Yeah I wasn't happy with Misty leaving the team too. Also the writers sure noticed it and feared a drop of the ratings making Brock come back for the Hoenn region.
Also the anime now deserves the decrease of the Ratings after showing us the Unova series. It is for me the worst series of all (even worse than the Johto series) and Brock's and Dawn's (and Misty's and May's) replacement weren't very good. Saying this and after 17 years watching Ash's journey in the Pokémon World I believe that Ash being replaced would really make the ratings drop significantly.
From the little I've heard of Iris's character, she seemed like she was like Misty (in that she wasn't of the female anime stereotype [look up Love Hina or see the Fiorello Fangirls in Princess v. Princess to see what I mean by that]), but never having actually watched Best Wishes, I can't make the call whether she truly is not of that stereotype or not, unlike May or Dawn. As far as Serena, never having watched XY obviously, I can't comment one way or another about her, though the romantic subplot seemed to be bad writing especially with the implication that she may have been the first girl his age, meaning Misty wasn't that.
And honestly, the characters aside (not because I liked them, but simply I have no way of gauging them as I haven't seen the series), I have to agree with you about Unova being the worst. I get it, it's supposed to be a reboot, but couldn't they at LEAST not have Ash mistake Iris for a friggin Axew as if he somehow got brain damage?
And I also am very irritated that Ash got the same ranking in the league that he got in Kanto. At least with Kanto, he has the excuse of actually starting on his journey. Ranking Top 16 in Unova, meaning he actually did worse than Sinnoh, was just terrible. Honestly, Unova now officially surpasses AG as being the worst league turnout (Say what you will about Johto, at least he actually increased a rank in that region, while he didn't increase in AG).
And they also did a lot of wasted potential in NOT showing a Pokémon Gym Leader tournament like in the games. I mean, not only would it have obviously given a much needed opportunity for Misty to return even temporarily (Brock unfortunately won't make it as he's not actually a gym leader anymore. Probably would still be one if he had done the responsible thing after AG and returned after Forrest blew it big time during Battle Frontier, possibly even after Johto after quelling the disaster in the first place), but it would have been a great way to advertise Black 2 and White 2 which did feature that aspect. Instead, they brought in Dawn who really didn't have any real business in the region. I mean, Ash doesn't interrupt his own goals to visit his friends no matter how much he might miss them, so why should Dawn basically get away from doing contests in Hoenn for Unova when it doesn't even have contests? At least with May, she actually had a reason besides meeting friends to go to Sinnoh, the Wallace Cup and all.
Plus, some of the attempts at Team Rocket being threatening, while it is nice they actually are treated as something of a threat, were, pardon the French, but half-assed. Take the arc where Meowth became a double agent, for example: Not only did it become somewhat predictable by the end of the arc that he'd probably betray them, but they apparently thought it was a "good" idea (NOT) to have Jessie and James act as though Meowth actually was fired despite there not being any evidence to suggest that they were being listened in on (I mean, honestly, if Ash and co were listening in from the bushes I could understand, but from what I could gather from what I read, Ash and co. were not anywhere near them at all, and how can they even bug them if they only just discovered what happened). You want an actual method of showing to the audience that the guy who is working with you is actually scamming you that works well, look at how Dr. Wily handled his treacherous plans for Mega Man, Bright Man, and Pharaoh Man in Issue 37 of Archie Comic's Mega Man.
The way shown here, the audience is in the know about Wily actually planning to backstab the heroes, yet at the same time, its done in a way that keeps the other characters in-universe in the dark in a pretty good way.
And I also wasn't particularly happy with how they decided to handle Team Rocket being threatening in the first Unova episode. I mean, yeah, I like they actually beat Ash, but having them beat one of Ash's rookie Pokémon would have sufficed enough, especially considering how their second appearance in the overall anime did have them losing quite pathetically to a rookie Pokémon (Ash's Caterpie, who was not only outclassed by Koffing, Ekans, and Meowth by a substantial amount in not only experience, but was pretty obviously just barely hanging on thanks to Ash's earlier foolishness of sending Caterpie off to fight a Pidgeotto and nearly getting it killed while Ekans, Koffing, and especially Meowth were pretty obviously much fresher).
Other than possibly Iris not being of the female anime stereotype, probably the only good things about the series is that we actually got a reunion with Ash's Butterfree after quite a while since a reunion was promised (granted, they still have yet to actually show a similar reunion with Pidgeot despite it actually being easier to find than Ash's Butterfree, but at least we've got Butterfree, as well as further proof that he didn't die, not that we needed the proof anyways), and that Iris, unlike a certain other female gym leader who left the show, actually was granted the opportunity to pursue her dream rather than being cooped up in a gym doomed to basically become stagnate and losing to rookie trainers and/or rookie Pokémon a'la JJM in the third episode of the overall anime/original series, and all against her will to boot.