Oh, I remember completely that whole mess with BW where Ash dipped down, and I also cited THAT as one of the reasons why, until SM, it tied with AG as the worst series ever, and alongside AG still remains the worst league turnout ever. However, let me remind you that when Kalos pulled that crap about heavily implying that Ash was going to win the Kalos league, combined with Generation VII not having any gyms with whether it even had a league being left in question, loads of fans on both sides of the Pacific ended up taking to Twitter in a storm and making it clear in no uncertain terms that this was a big mistake of the writers (not to mention at least one of the production staff also made it clear such was a mistake). The writers would NOT want to repeat that mistake lest they incur even MORE losses against them. In fact, part of the reason why Misty and Brock came back in SM was suspected to be due to a similar twitter outrage over their being excluded from Movie 20. And besides, unlike with prior regions, this is literally going to be the first annual league for that region. Having him lose that, especially when that league is literally the first one of the region, would actually reflect VERY badly on him, and would likewise cause an even WORSE outrage than the Kalos events. I know if I were the writers, I'd make sure Ash wins the current league, even if it means the end of the show, because I'd be deathly afraid of incurring another viewer outrage via Twitter if I did otherwise.
And if they're going to continue the show even if Ash wins the league (and let's face it, he'd still need to face the Elite 4 and Champion before he can come close to achieving his goal anyways, and if he can continue his journey despite winning the Orange Islands league and the Battle Frontier, the latter of whom had trainers comparable to the Elite 4 if not rivaling them in skillsets, he most certainly can continue his journey if he wins a league), they'll need to focus on making sure Ash receives loads of development as well. XY gave him some development on that front, so it's not impossible. Heck, even AG at least did some development for him by actually ATTEMPTING to take people under his wing. One of the ways to ensure that development is to actually have his dad debut, period. And don't claim they took too long. Don't forget that Brock's mom took nine years since her first mention to debut in the main series (and the dub even implied that she was dead), and even Jessiebelle got an encore appearance over a decade after her initial debut during DP. If they can bring those guys back/debut them (and they weren't even fan favorites anyway), them bringing Ash's dad in isn't impossible at all. Sure, maybe improbable, but then again, Jessiebelle reappearing in Sinnoh wasn't exactly probable either.