I think that people are angry not because of the happy ending, but because it seems too rushed. In my case, at least, it's like this. Gege introduced a lot of things/characters without going into detail(idk if I missed something, but itadori's domain appears literally out of nowhere, for example). The premises for this shonen were good, but the execution... Simply could've been better. I've seen a lot of stories that didn't go like the public wanted, but that were still reviewed as very good. I don't think that the problem is how people wanted it to end, but more how it should've ended. Obviously this is just my opinion, feel free to disagree(〜￣△￣)〜
I think that's valid but no way that people's opinions didn't effect Gege in any way with it being so popular. The rushed ending and it being happy could literally be them being done with the media's bullshit and giving up in a way that they have accepted that they are pressured to do good and can't outdo people's expectations. And either way with how people wanted it to end and how it should've ended could've been the same or it's just nonexistent to Gege at that point.
Mainstream animes like these gets shit on a lot either way because it means no one will ever be satisfied. Everyone alive will be too happy, everyone dead will be too unsatisfying. Anything Gege could have done would not have reached the full potential expectations everyone would have. Kinda sad, anything they do will probably be always based on people's wants and not their own, making them lose passion on how they started in the first place