Roxas at the end of KH2 accepted that he'd go back to Sora. Sora tells him that Roxas deserved to be his own person, that he didn't deserve what he went through, and reaffirmed his right to exist as an individual, which is what Roxas wanted in days. This pays off in kh3?
Well, as much as I love them, what exactly are they going to do going forward? Their stories are effectively over now which is why they should’ve stayed gone. I remember someone told me “if you have to think of a reason for a character to be present, they probably shouldn’t be”