Has nothing to do with "Realism". How many people die in the game especially by the player's hands or those who they helped? Would you classify their lives as ending Happily?
You're exactly confusing realism and fiction here. From the realism POV, it all sucks cause lots of people died -- true. From the fiction POV, those NPC deaths are supposed to add meaning to MC's arc, yet this arc is flushed down in the end.
Just from a perspective of the game it rarely has a happy ending, even the setting makes this well-known from the prologue. (Suggesting it is more common ending in NC to have a bad ending than it is in our current world.)
This thread isn't to complain about the lack of a happy ending. It's just that the endings that we have make the rising to falling action pointless. I'd say they are not even pure 'endings' -- they're a 1-hour "climax + falling action + poor resolution" attached to a 10-25 hour exposition+rising action.
And speaking about bittersweet endings, there is one million possibilities listed by all the users on 700 pages earlier.