And yeah, it would have been nice if a lot of it wasn't spoiled by the trailers. But, like, welcome to Kingdom Hearts? The trailers revealing too much has always been a thing. Don't know why anyone would expect different at this point. Let it be a lesson now, if you don't want a lot of KH3 spoiled, limit how many trailers, interviews and scans you see.
Trailers, interviews and scans are created in order to make us curious and excited. I'm not sure that blaming the fans for actually watching and reading things which are created FOR them is useful. Maybe the creators should reconsider what dialogues and footages they want to show us. And at any rate the problem is not that the trailers spoiled too much about BC (because honestly they didn't)... the problem is that BC didn't contain much story to begin with. :'D