I would typically tend to agree, but I've followed the complete trainwreck of business practices with the brand under Disney. There are a few flukes here and there and some hidden gems, but for the most part, Disney has done nothing but play things safe, especially in the spin-off material that might dare to include Luke, Han, Leia and any of the major film characters.
As for why we SW fans would dislike it? Technically, if it gave us a good story with Luke as a party member, maybe even helping train Sora to become an even better fighter and having a great dynamic and chemistry with him, we'd jump all for that. One of my favorite elements from the old continuity was Grand Master Luke and his New Jedi Order, where he took his niece and nephews under his wing and helped to train them, not just to become Jedi, but just to become better people in general.
That'd be the kind of dynamic I'd like to see. Two paragons working together. But I'm pretty sure that's not what we'd get and we'd no doubt get a lackluster story that wasn't allowed to take any chances either.
As a fellow Star Wars fan I can see where you're coming from with these worries, but I doubt that Square and Disney would let the Star Wars executives meddle too much.
If it will be considered a non-canon instance of Star Wars anyways and only include a stand-alone adaption of Episode VI I doubt Kathleen Kennedy or the Story Group would even care to be included because it doesn't affect Star Wars itself in any form.
So if the story falls flat it won't have anything to do with the writers on the Star Wars side.
There already exist non-canon adaptions of the main films like the Infinities anyways and the KH incarnation of those would only be yet another filed under the brand.
This would certainly deal with the whole "Star Wars is to big and full of its own lore"-problem because the KH-incarnation would be just a small snippet of its own that borrows a part of the Star Wars story without diving fully into it.
And as Absent mentioned, considering the overall themes Episode VI can stand on its own and serve as a tie-in to the overall KH story without having to dive too deep into SWs own.
Darth Vader is a peak example of a redeemable villain while the Emperor is a peak example of an irredeemable evil and how there can be differences among "evil doers" despite their actions sometimes being pretty similar.
I wouldn't even be surprised if Nomura uses this for his own ends in order to show Sora (and the audience) that for example Xehanort actually is a "Vader-type"-villain and not an "Emperor-type"-one despite all the evil stuff he did in the first saga.