Here's a legit reason to change the ending. It completely destroys everything. It destroys the messages, the bonds, the plot, the sub-plots, the characters, it changes genre, it's poorly put together, it makes no sense, it's insulting to everyone that followed it, it doesn't resolve anything, everything that was supposed to be addressed is just forgotten, characters go 180 on themselves for no reason and the list goes on and on.
Oh and about Kishi losing credibility and he will be hated by the whole Naruto fanbase. Guess what he already lost that credibility and he's already hated because with this thing he did. He just told us that the events in the manga that he wrote are not completely irrelevant and don't matter actually they never even existed. The manga was just us imagining things. The movie is the real story, everything before that meaningless.
I agree with what you are saying, I do believe the whole Naruto falling for Hinata contradicts a lot of Narutos actions in the past, like loving actions towards sakura and also only falling for her because of retcon scenes in a movie. Lots of characters were butchered and development made void, and also scenes and development should have been put into the manga for naruhina to make it more credible. And when I say development I mean romantic inclinations for hinata on Narutos end, not more scenes with Hinata 'Naruto-kunning'. Yes, kishimotos credibility has fallen for a lot of people but again there are the people who actually did like the ending/movie and still like Kishimoto. But eh, I don't care about this pairing anymore after everything kishi and SP have done.