It's a super-high bar for me to make me want to quit the series. Frankly, I don't think I have a bar—I don't think anything could make me stop playing.
However, if anything could, it would probably be one of these following conditions:
1) Nomura died/quit and someone with much less artistic vision took over. Tetsuya Nomura IS Kingdom Hearts, and Kingdom Hearts IS Tetsuya Nomura. Frankly, it's more him than it is Disney. His vision, his narrative and aesthetic style, are inseparable from the identity of the franchise. If he were to leave the picture, I doubt that anybody could fill his shoes. For all the flak we give him (and much of it he deserves), I can't lie that he is one of the most unique and unpredictable storytellers that I've ever come across. This series would never have gotten off the ground if not for his creativity and ingenuity. If not for him, this series would have been probably one game in length, and it would have just been Disney characters bumping into Final Fantasy characters, nothing deeper than that (and that would have sucked imo).
2) If the games became routinely dull. I mean the gameplay specifically. Even if the story continues to be as insane as it is, if games like Union X and Dark Road prove to be the future of the franchise from a mechanical perspective, I don't know if I could stomach that. Even a console game that isn't fun to play will have the same effect. Half of the series' appeal is its gameplay, and the other half of its appeal—all the narrative and artistic stuff—is quite hit-or-miss at times. I would probably keep up with the story, but my desire to actually buy and play the games would be severely diminished if that ever happened.
3) If they focus exclusively on current Disney movies for worlds. Don't get me wrong, they're fun, and I know that part of the point of these games is marketing for new Disney releases, but I would hope they keep the practice of including at least a few old Disney movies per title. I don't care if they're Renaissance movies or Classic movies or even movies from Disney's intermittent dark ages. All the Disney movies that give me that feeling of nostalgia are the ones that came out before I was an adult. The new Disney movies just wouldn't give me that same glow, which means playing through their levels would be a bit more of a slog. I appreciate that KH3 kept Hercules, PotC, Winnie the Pooh, Toy Story, and Monsters Inc as new but not-too-recent titles. That helped strike a balance in terms of target demographic—there were worlds there for young kids who grew up on Frozen and Big Hero 6, and older adults who were watching Toy Story and Hercules when they were the same age.
4) The characters continue to be poorly written. I'll defend KH3 to the death from its detractors, but one thing I won't defend is the writing of the original characters. Sora wasn't bad, but too many of them, especially Aqua, Riku, and Kairi, had really dull dialogue and didn't have any chemistry with the other characters like they used to in the older games. And part of that has to do with Nomura not knowing what to do with his female characters, which is continually infuriating. But if this goes on for very much longer, that might sour the series for me.
5) Going too hard on the fanservice. I like a little bit of fanservice now and then, but doing a lot of it really takes me out of a story. And KH3 had a huge problem with that. Actually, this is probably the one thing I think ReMIND made WORSE. I get that KH2 was the golden age of the series, but there's a limit on how many call backs and references you should have. And the constant hyping up of the same old fan-favorite characters has to stop. We get it, everybody loved Roxas and Axel when they were young, but playing off of that too much comes off as artificial after a while. And making a bunch of gameplay callbacks in ReMIND to KH2 was fun as well, but I'm hoping the devs will actually experiment going forward instead of trying to just recreate KH2's mechanics every single game.
6) Exposition instead of showing. This is a big one for me, because it's so bland and there's little excuse for it nowadays. Easily one of the worst things the series does is have people stand around in a place and just talk and exposit with very little movement or action or even interesting cinematography. This was forgivable in the earlier games because they were on older consoles and the series wasn't as big so they didn't get as high a budget. But now, KH is playing with the big boys and is getting super-high bucks, so they need to start doing better visual storytelling. This practice of just having people stand around a room and exposit to each other was painfully noticeable in KH3, and I don't think the series can continue being like that going forward. It just looks and feels terrible.