This proposal pops up like every three months or so somewhere in the periphery around the KH series so it is nothing new, yet the core issue remains the same.
The Disney characters have the advantage of already being established, iconic characters with fully realized character arcs. The problem isn't that (a mass of) original characters are there, they're just more often than not poorly developed and at a trickle-drip's pace.
Indeed, although the original characters being underdeveloped and not getting enough focus (apart from a select few such as Riku and Xehanort) is not primarily the fault of the Disney properties and characters themselves but how Nomura and the KH staff are distributing the available space in a a single game and how they lack the creativity and sometimes boldness to effectively combine those aspects to enrich both the original and Disney characters, but that is a problem that predates KH 3 itself.
The combination aspect may be however more recently indeed partly to blame on Disney due to their, as Spockanort mentions, somewhat hamfisted approach to their properties in the past few years.
Seriously? The Frozen world in KH 3 was so underused and, for lack of better words,
unimportant that the game could have functioned much the same without it.
Imagine if we had a visit to Radiant Garden instead of Frozen which incorporated a) Saix as a world organisation member and boss battle, b) assistance from the FF cameos, c) involvement of Lea and Kairi as Guest party members, d) cameos of Tron and the Chamber of Repose as another link to the "save Aqua" plotline as well as gain more info on Xemnas and Xehanort and last but not least e) a small side plot on both Kairi's childhood and Lea/Isa's past to set up future plotlines for them without resorting to mere exposition cutscenes during the final battles.
The problem is not the Disney properties taking too much space, but how they are used, or rather
not used, same as it is with the original properties including the characters.
It's not Disney stealing their thunder, it's Nomura and co. not putting enough care into coherent scenario writing, connections via world building and characterisation letting all characters (original and Disney) playing off each other and letting them develop via that.
Because big plot arcs with grand flashy gestures (and battles) as well as twists and "surprises" are more important instead of fleshing out stuff.
It jumps only evermore into one's eyes recently because with Disney exerting tighter control over their brands some Disney parts of the KH series
feel even more detached and superfluous to the KH-verse as a whole.