Actually, I think it goes deeper than that.
My understanding is that US culture is reluctant to accept or endorse or approve of movies that do not have a happy ending and that do not leave the audience in a 'feel-good' mood on leaving the cinema.
That means that movie makers, directors, and producers seem to be increasingly unwilling to risk making such a movie, and that any ending where matters are nuanced, or a bitter-sweet note is struck, meet with audience disapproval.
Now, I get that movies are a form of escapism and that nobody wants to be reminded of the messy compromises of real life all of the time. But a culture that insists on fiction supplying a happy ending all the time does itself a disservice, and - in essence, censors its artists from exploring the human condition fully.
Reverting to Star Trek - and STNG - which, at their best were excellent dramas - the very, very best episodes tended to have that downbeat ending, striking a bitter-sweet note of rueful contemplation. The very best episodes were so good they actually hurt when you had finished watching them.