The First Law of Bad Literary Novels is simple: there are no happy endings.
It’s the same story with Big, Beautiful Movies with Sad, Stupid Endings.
Now, that’s not to say every book and movie needs a prototypical Hollywood happy ending. Tragedies should have sad endings, and there are plenty of classic movies where the ending is ambiguous.
Rocky actually loses his first fight. The victory comes from not getting knocked out – and from the journey from loser to contender. Rocky suffers, sacrifices and grows. That’s why the movie is good: there’s a big contrast between where Rocky is in Act 1 and where he ends up in Act 3.
The trouble with these movies is the audience doesn’t want to see them again, if they ever saw them in the first place, because the ending sours everybody, despite the beautiful imagery and amazing acting.
I’m not saying you can’t make a great movie without being low-brow and throwing in more explosions than Michael Bay ever dreamed possible.