I think everyone understands that tv shows aren't real life, but "shouldn't reflect that"? Why not? And why especially sci-fi fantasy?
Hell, folks here knock Trek all the time when it portrays events they consider unrealistic.
I have no real problem with the idea that characters should always have happy endings, meaningful deaths, etc., but every once in awhile, especially if it's written well ("The Visitor", to a point), I like the reminder that not everyone gets to have such things. Especially given Trek's penchant for trivializing death by killing legions of redshirts but still having happy endings for everyone "important".
Sam chose the fate that he most wanted for himself, and I don't think that qualifies as an unhappy ending. How many people get to do that? Is what we want for him supposed to trump what he wants for himself?