I wholeheartedly agree. Do you think it really came down to Nomura's saltiness over Versus XIII? Or do you think he was tired of them? The thing that really confuses me was this quote from an interview in June 2018. Sorry, I'm really bad at linking these things! Newbie here!
"I can't disclose too much about the Final Fantasy characters or whether or not they'll be in Kingdom Hearts 3, because I can only speak to what's been shown. But the reason you haven't been shown any Final Fantasy characters is that there are so many original characters that need to resolve their own problems and issues. And because their issues and problems are very difficult, there just hasn't been that much space to fit Final Fantasy characters so far.
When you look back on the first Kingdom Hearts, Sora was still a new character, so we kind of had the Final Fantasy characters as supporting characters. But now that it's been 15 years, the original characters are more polished than they were before, so I don't think think they need the Final Fantasy characters help as much as they did before. [...] Also, when we first released Kingdom Hearts, there weren't that many other titles that brought together the Final Fantasy characters. I actually think that Kingdom Hearts may have been the first to do that.
But nowadays, there are an abundance of titles that do that, so I don't see the value of having that kind of feature in the game anymore. But that doesn't mean that I'm saying that there won't be any Final Fantasy characters. Please look forward to future information."