but really KH3 had no plot besides the sequel baiting
Disagree, but I do think I see where you are coming from.
The issue with KH3 is that from even before it started we all knew 100% where it was going to end up. We all knew there was going to be a battle. We just didn't know who, exactly, was going to be on what side fighting who. But knowing roughly the end game before we even started the game itself caused a bit of a strange dilemma where people dismiss (understandably) the entire first 70% of the game as 'filler' because "we all know where it is going to end up, so why is the game dragging its feet getting us there".
Now people can disagree or not whether this was effective or if they liked it, but what I believe Nomura and the team tried to do is 1) make sure that everyone was caught up on the current standing of all the characters, so that they could understand who they were/where they came from in the final battle (which is why they spent so many cutscenes talking about the hearts inside Sora) and 2) give Sora character development.
The way I see the game, the first 70% of it isn't actually focused on
main story plot, it's actually supposed to be a personal journey for Sora. It's about him learning to overcome his first real experience with self-doubt, feelings of failure, and somewhat importantly: independence and realizing what it is that he wants to fight for for
himself. The game is about Sora's own journey in growing up--a very different one that Riku had, but still a one that it is very important he undertook. These are all themes and ideas that he explores throughout the disney worlds, and even somewhat through the graveyard during the final fights.
For so long a lot of people were ragging on Sora for being a flat, unchanging character, so I think they listened to that criticism and decided to focus on him and give him some deeper thoughts and feelings beyond "friends good".
Where the issue lies is I don't think many of us were going into KH3 and expecting the build-up to the Keyblade War to be 90% a Sora-only focused personal development plot and were sort of missing that memo as they played through, wondering when the "story" was going to start--not realizing they were overlooking the story the team was trying to tell the whole time.
To bring this home to your larger question/point:
This is why I think the "sequel baiting" is very valuable. KH3 was Sora's journey, at the end of the day. But now he is gone and if there was nothing in the game hinting towards future development, people would likely be thinking the series is over. The questions that were raised are interesting because they
involve other characters apart from Sora. As much as I love the boy, it will be nice to see other characters get some spotlight and development as well. The sequel hooks seem to suggest various places where that is going to happen and this is what I think excites people. It is fun for them to be able to speculate, or get future games about their favorite characters who up until now maybe only had a small or limited role in the game series. It shows, at least in my mind, that future plot points are possibly going to be more balanced, and involve a more ensemble role than just about Sora and his development. I am excited for that, even though Sora is 100% my favorite character-- there are others I care about as well and would enjoy learning about more.