I would imagine that Square & Disney already have a set contract for KH3 & Disney can't really tamper with that even if they wanted to.
Kingdom Hearts is such a peculiar case for Disney. It was the only means of keeping Disney Animation relevant as the early 2000's were all about Pixar. I think that Disney is thankful enough for the Kingdom Hearts series to let them have free reign. And unlike Disney INFINITY: Kingdom Hearts wasn't bandwagoning on a trend (*cough* Disney Crossy Road *cough*). There's too much already invested in Kingdom Hearts to axe it.