Uh the hand held market is not dying. 3DS might have peaked but it's still seeing numbers strong enough to stay in production despite the Switch existing.

On top of that while yes, the handheld games are more or less easier and faster to make, the biggest reason so many KH games released on the handhelds was because Japan has an insane mobile gaming market that is at this point borderline part of their very culture.

That said I don't think we will see many non numbered titles going forward. It seems like Nomura is being shifted to all Kingdokm Hearts all the time. His time between KH2 and KH3 was largely split between Final Fantasy and Kingdom Hearts and SE would force his priority to be on FF. With his removal from working on FFXV, that means his only FF project is FF7R. I hope thgat following that project he goes back to being an FF character designer and steps down from th e director position so he can focus purely on KH.