The Grey Nayr wrote...
1. If there was a perfect happy ending, everybody would go out of their way to get it. They'd ignore everything else and go for that alone.
2. The theme of Mass Effect is actions and consequences. Having a perfect happy ending negates that.
1.People play games to have fun. If the writers provide a sad ending, a bittersweet ending, and a happy ending then, everything else constant, we can assume people will choose the ending that suits best their tastes.
If we assume most players choose the happy ending then the logical conclusion is that such an ending is more fun for them. Then, why on Earth make a game deliberately less fun for a significant part of your audience by depriving them of the option they like best?
2. I would argue that there isn’t a single central theme in Mass Effect.
For instance, in ME3 the writers invite us to choose between Sacrifice and Unity at the very beginning (when Shepard is asked how to defeat the reapers by the defense committee), and later near the ending we are asked to choose again when Shepard gives her final speech, (again focusing on sacrifice or the “ties that bind us”).
Now the game fully supports this; you can forge a powerful alliance of peoples by uniting the galaxy with sacrifice only playing a secondary role in the game if you choose to do so…. Only at the ending is this completely reversed and sacrifice, (of a mostly pointless variety, imo) becomes the only acknowledged theme….