Honestly? its a little bit of an exaggeration, but I do suspect it'll still be a few years before we see anything about it. This is Nomura we're talking about, and Disney to boot. Nomura loves to change his mind and insist on whole new directions for games. And Disney is very fickle with what IP's they let others use. Rumor has it that KH worlds are basically always in flux because Disney keeps changing its mind.
That's a combination that spells mega delays.
Is the Nomura thing built from when he said he thought about turning Versus XIII into a musical because he saw Les Mis? Because other than that I don't know where him changing his mind and insisting on a whole new direction comes from, because I've not seen anything like that affect Kingdom Hearts.
Not sure where that rumour about Disney came from either. They suggest worlds, Nomura suggests worlds, then they work back and forth on establishing the look, handing over data and references, getting in contact with the voice actors, etc. Worlds get cut, but that's the same of any project.
Nomura did say the process took a little longer with Disney for KH3 than previous games because they had the creatives who made the movies still at Disney so things had to be on point (I remember eye lines were an important thing for Square Enix to get right) but all that paid off in the end since the game is gorgeous and replicated the look and feel of the movies so well. They have that experience and carried that forward with them to KH4.
It took 5 years and at least one engine change to go from the announcement of 3 to its release. I imagine at minimum it'll be 5 years before we can even think about playing 4, which is 2027. On top of whatever delay happens because of the mobile game, which is also facing its own delays.
Is 5 years of development a bad thing? I thought it was about the standard for an AAA game this time. And you seem to be counting 5 years since announcement instead of 5 years since development started (2020).
I said earlier, they've started development of KH4 with an engine already in place, one they know, and then upgraded to the next version of it. KH3 started with them doing gameplay tests in UE3, tried Luminous, then scrapped that and retooled for UE4 in October 2014. It was in development until it went gold in November 2018. KH4 has been in development for over 4 years now. If they take another 3 like you suggested, then that shows there's some deeper issues within Square Enix, especially when compared to Rebirth's timeline, and how confident they are that Part 3 can be developed in the same time frame.
I don't think they'd have KH4 be beholden to the release of a mobile game either, it's like saying Rebirth should have waited for Ever Crisis to have done the First Soldier storyline before releasing. Besides, Missing-Link is about to have a beta test this week and it is coming this year. I don't think it will last long but I really don't think a mobile game will hold KH4 up.