WARNING, long post ahead:
I highly doubt XY saga is last series for Ash. For several reasons.
From business perspective as long new games keep coming out with anime serving as one of main tools through which new pokemon and generations are advertized there is no reason to believe how series will be concluded. Unless they experience such drastic decrease in ratings and people interest that entire point of their existence of telling story about characters, their attempts to achieve dreams in pokemon company and marketing value becomes pointless, no longer being able to adequatelly promote new products.
As far as i know ratings while shaky and nowhere near to previous days when pokemon was bigger hit (longevity, some of unpopular decisions or quality of writing may have contributed in decrese over time) are still big enough to warrant continuation.
Now some may say why not introduce new character and replace Ash?
Because of several reasons actually.
Reason
number 1:
Pokemon was always intended to be treated as series which is centralized around one main star(Ash) following his journey of becoming master and lifepath of his friends , allowing generations to grow up with one representative and story taking place in same timeline and universe. Because of this pokemon series became so recognizable and iconic worldwide, achieving such popularity.
Similar how Naruto, One Piece and similar anime do following one consistent storyline and while they dont cycle through characters keeping everyone and have much stronger following, one big similarity pokemon has in common is that all of them have one character as show main face.
I suppose perhaps such approach could have see light of day if Ash was replaced after Kanto or at most Johto with anime going through complete reboot, but after being driving force for more than decade and a half he became too integrated within anime and iconic with pokemon becoming too associated with his character and viewed as one big continuity.
Doing it now is too late with people being used to this formula, so new approach would probably end up as fiasco .
Not to mention make no sense to replace successful character who didn't even reach apex of its true potential after so much time.
Other shows like Digimon, Power rangers, Yugioh etc are one of examples which shows how ditching main face and iconic representative of some anime can backfire negatively on series positive image they built around themselves. Such examples experienced big drop in people following and respect for this animated shows losing notable portion of loyal fandom not being even shadow of their former selves in popularity anymore.
Nonetheless in comparison to main stars of aforementioned anime Ash has been constant, permanent driving force of pokemon for last 16 years. If writers ever wanted to replace him and bring his joiurney to end in favor of successor continuing on, logic implies it would happen by now. But it didn't because keeping same main star treating it as continuity for long enough to leave same impact on upcoming generations, cause fans to start associate certain characters with one show viewing it as its brand which helped better to promote something worldwide.
Plenty anime have permanent main stays not going through complete revamps after every few years, and they are usually more popular than shows which constantly cycle through characters . Why? Because it brings benefit that audience has someone to who it can get attached investing into character without fearing that he will leave soon, making them interested in following his journey for long periods of time, going through new adventures and learning more about his past, motivations and aspects of his dream. Although that only applies to Ash not staying sequential with his companions.
But regardless of that Ash alone serve as pathos, ultimate driving force providing positive thing in sense of anime with long term stay characters containing mascot,main face which stand out for something. Someone with who people can associate pokemon series being able to connect with such type of character. Its good for advertizing, imbues anime with continuity bringing established storyline on which writers build up and has iconic character which represents certain series ensuring that anime will have easier time in attracting new generations of fans toward it given how there is someone which represents them.
Reason
number 2:
Current status of Ash progression as characters and story hasn't met status of allowing to bring his journey to end, tie up loose ends and lead toward proper, long lasting closure. XY saga alone with way Ash is currently going forward not fulfilling requirements or giving enough time to bring this character journey even close to such objective.
He hasn't won a league yet,concept of champion league which was for first time brought in DP is highly unexplored and Ash never officially challenged E4 and champion.
From character point Ash is still quite brash and impulsive having several flaws which are standing on his way of achieving goal rushing into things without thinking,having problems in utilizing power of his pokemon to their true potential, being at times overconfident overestimating his abilities as trainer costing him matches in process etc. Which was showed to be case at times even in DP with relatively easy defeats from Palmer, Flint and Bertha showing he is still far from elite strength having room left to grow. Never mind unexplored backstory being ironic how little we know about his past.
We also cannot deny fact how its completely uncertain what other steps someone may need to accomplish after winning league or battling E4 with whole concept of pokemon master goal being for large part still undefined with little clearer picture not being given until recently in Sinnoh.
Who says Ash journey would even have to stop after winning pokemon league and battling E4? That wont make him master, nor best trainer in world with there existing other regions and leagues to explore, rivals to beat, new pokemon to catch, enter various side quests with harder and bigger challenges like BF or Orange Island championship were to learn and prove himself as trainer. Some sort of master league as final step on his road of coming close toward ultimate dream. Which would be fitting for grand finale.
We could also learn more about Ash past, family(like his parent), do more with ton of older pokemon which are rooting at Oaks lab etc. Possibilities are endless, and Ash can definitely grow forward as trainer having several flaws which need to be perfect, along with creating new obstacles.
Another very important thing is that all of Ash past friends and travel companions are left unfinished with plots which form their stories, connection they have with Ash in affecting his growth and lifepath along close bonds which connect them(or at least should)are left at pending status untied.
Like May and Dawn which never won GF or overcome rivals, obstacles standing on their way? Misty whose water pokemon master goal was left unresolved and desire to reach expertize and level of strength like E4 mermbers have as water specialist(as her idol Lorelei)not being cleared up in what direction her story will go in future and how will things with her sisters and gym play out, nor coming close toward her dreams?
Brock who is left to study to become doctor not coming closer toward it?
Cilan who didnt become S class connoisseur or Iris who wants to become master of dragons?
Etc, etc.
There is surely a whole lot of loose ends and open things to bring to conclusion before writers even plan to bring Ash journey to an end. Unless they will just cut it abruptly leaving everything unresolved and ambiguous which would not only be underwhelming but disappointing for such long running show like pokemon too deserving to have believable and meaningful conclusion bringing whole journey to certain climax.
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Lastly presence of Serena and her interest toward Ash in something which appears to be more than friendship dont play any say in how long Ash will stay. Depending on how you interpret it Misty harbored romantic feelings toward Ash as well, yet it played no difference in his staying power. (Many who support this would say she still does using certain hints from hosos like rejecting Gergio date because "there is someone else" as ammunition how this aspect of character was left uncleared too).
Ash continued to freely travel across many regions ever since Misty left with one sided thing from her not posing any obstacle or obligation that writers end Ash journey.
Same can be expected with Serena because oblivious and immature as Ash is having only battles, friendship and trying to become best on his mind, it is unlikely to expect that he will all of a sudden start returning feelings back toward girl which may develop crush on him. With love subplot not having much place in series like pokemon according to blogs from one of head writers who was in charge for OS(Takeshi Shudo, rest in peace) stating how expansion on such plot would only take away from what main message of pokemon series is. About journey, friends which explore world and support each other in working toward dreams and pokemon themselves.
Simply because of anime limits and struture under which things are written in this show lacking deep enough plot and complex development so that romance could play vector for significant character development pushing main story forward.
Very high probability of Serena getting replaced for 7th generation with another girl(sadly number of abandoned companions uncontrollably increases) doesnpt help either, with any started relationship and story in love sense not having much future in such situation.
This small passage from interview with Masamitsu Hidaka(pokemon director until Battle Frontier and from than continued working as storyboard artist) from 2008 on pokebeach is another reason why i highly doubt how Kalos(or known to some as XY series) would be Ash last saga spent in anime:
Now an overall question about the show. Will Ash ever become a Pokemon Master? He laughed and fell back on the couch. He blatantly said that when Ash becomes a Pokemon Master, the show will end. It will be the last episode.
So unless something drastically changes from writing staff taking plans for future of pokemon anime in different direction, its reasonable to expect how Ash isn't leaving nowhere, until pokemon show itself ends for good(real ending with no new adaptations happening afterwards).
p.s.In response to OP post. Im not sure how exactly Brock will settle down with prof. Ivy when ever since he left her(its heavily implied he was dumped), she was never brought up again. Let alone Brock maintaining contact with her. Relationship between Misty and Tracey is on shaky legs too given how Misty never hinted to hold anything other than good friendship toward this character. And viceversa. So i wouldn't say anyone in this show found someone to settle down , rather not existing canon relationships so far (if ever).
