The Osaka team is hardly the only one that develops KH games. There's no reason not to have other games, and it would be really disappointing if the series moved straight into KH4.
Osaka team is nowadays considered _THE_ KH Team and they ARE the only one developing console KH games (home or portable) at the moment. Whatever is left of the original one that made KH1 & 2 move with Nomura from project to project, I think (at least did with Versus XIII, not sure if they are still with Nomura on FFVII Remake). Jupiter (GBA Chain of Memories) is nowadays a Picross developing machine (IIRC, I think duckroll or someone mentioned that Square Enix basically poached everyone who worked on The World Ends With You to work for SQEX after its release, so the people who made KH :CoM for GBA are probably at SQEX already) and h.a.n.d. is a company that either develops smaller games (especially during the DS era) or only support the development of bigger ones in some capacity. I don't think they have the capacity to handle the core development of a current gen level game. And so far, SQEX has kept the development of KH games that matter (outside of GBA CoM) as in-house projects. They don't have the workforce to work on two console KH games simultaneously (even if one of them is "just" a Switch game). Even during the PSP & 3DS days, they only had one big KH game under development at once and the two other games were throwaway DS games that probably had, like, 1/10th of the budget & scope, if that.
Kingdom Hearts is a somewhat established franchise and they have some pretty strict ideas of what the games should be like in some fronts. They don't want to hand the bigger games to just anyone. Even Osaka Team only got to their position because Nomura was so pleased with their work on Re:Chain of Memories and even then the Osaka Team was assigned a bit smaller scale spin-off games at first while Nomura was still holding on to the idea that the OG KH Team would return to the franchise with KHIII after Versus XIII. It wasn't until around/after DDD's release that they "officially" upgraded Osaka team to be THE KH Team, who are entrusted with the future of KH for the foreseeable future.
So the only option left is outsourcing, because it's doubtful they'd waste the Osaka Team's time with spin-offs that take 3-4+ years to develop (no matter how important to the overall storyline), but SQEX isn't about to hand over the more important, bigger KH games to just anyone. h.a.n.d. isn't big enough to handle the core development of a mainline level KH game. Maybe Platinum Games but I imagine SQEX (& Disney) don't necessarily think that they are a good fit for KH. They found success with NieR Automata and I imagine they want to keep doing those kinds of more "mature"/darker collaborations with PG rather than putting them on a more kid-friendly franchise like KH.
And we aren't moving straight to KHIV. We've got the LOVELY mobile game that is pushing forward with the narrative that will eventually make the basis of what we have to deal with KHIV (and is already pretty heavily into setting that up).