Will they ever release after KH3 games again on handhelds that are important to the story?

I rather they keep releasing it on one console

I don't think so. The PS4 is perfectly able to handle future titles.

It's Square Enix. So probably

 

But I guess it depends if there are any popular handhelds in the future. If so then for sure.

Square Enix tends to support the handhelds pretty well, but they took a hard pass on the Vita for a lot of things, including KH. Depends on the state of the handheld market. Sony's probably going to wait a bit before rolling into their next one, Nintendo's got the Switch but still inexplicably support the 3DS, so we'll see.

I think they'll just keep updating union cross on smartphones after KH3 to give more setup for KH4

There won't be any handheld to make games for. Handhelds are practically dead, smartphones are the future.

 

The next KH game will be on console for sure. It's the start of the new saga. They will also use assets of KH3 development again, which will shorten the development time.

Not unless Nomura wants another Kingdom Hearts handheld game. 

It's most likely that after KH3 we'll get KH4. (So around 2030) And KH4 will probably not be on a Handheld.

Probably, though if Nomura was smart he'd make the side game smaller downloadable games you can get on any console

 

I don't think so. The PS4 is perfectly able to handle future titles.

The PS3 could handle the games too. That's never been the issue. Making games for handheld is significantly cheaper. A console game with the fidelity the PS4 )or higher) asks for is a major investment, so a less important game could still be relegated to a lesser system.

 

There won't be any handheld to make games for. Handhelds are practically dead, smartphones are the future.

The next KH game will be on console for sure. It's the start of the new saga. They will also use assets of KH3 development again, which will shorten the development time.

the Switch has already been a massive success and despite that Nintendo has said they have plans for a 3DS follow up.

Handhelds are still super popular in Japan beyond that, and mobile gaming as a concept is ruined. There's no home for a decent budget game, only predatory gatcha games and microtransactions. It's not the future.

Well, Square always does things so that they can cover their bases on a multitude of consoles, so it's not out of the realm of possibility that we could get more handheld Kingdom Hearts titles in the future, but maybe for the next saga, Nomura will want to keep all installments on PS4, or, if he decides to start the next saga on PS5, then PS5, ya know?