You have to feel that it's a good thing when Joseph Gordon-Levitt kills himself. Since he is the protagonist, and in the vast majority of movies the protagonist is a vicarious fantasy figure, this is an unexpected and difficult demand on the viewer. There is an up ending, but it's for the kid and no one (except maybe some confused children accidentally watching this film?) identifies with the kid. Many people find it very difficult to empathize with fictional characters they don't identify with.
The movie is not really concerned with a dystopia, nor does it carefully think through the time travel. It should have, and would have been much more effective if it had.