Hi, I'm new, so don't mind if I join you, I hope!
TL;DR: I don't think it's a big deal to continue to wean off FF characters, since Sora, by now, is more grown and able to wield much greater power. But, at least a cameo of all of the FF characters would be nice.
In terms of the rules for marketing games and anime in Japan, you should usually "solve every problem" in sequels that follow - therefore, should that require less help from supporting characters, it should be done to wean them off to avoid creating too many more issues in the series.
Much of the KH series has been about solving different problems, and leaving the main problem "Xehanort" to the main series. Using FF characters too much would cloud (no pun intended) Sora's judgment and dexterity in problem solving, and make things too hairy for the situation.
Let's not forget, they (the main characters) are not "kids" anymore, they are grown and ready to tackle things themselves. By now, Sora's powers are great, he should be able to manage.