It's pretty upbeat all things considered. We know there's a lot of folks who survived like Jane Valderama, Zack and Bobby, and a bunch of others abducted during the invasion. Plus all those historical homies like Jane Austen.
It's kind of hard for me to get a depressing vibe from the ending when the whole universe, past, present and future, is now open to the Saints. Not fixing the Earth because they're too busy blowing stuff up with Attila the Hun or saving mythological creatures on Noah's ark is exactly the kind of stuff I'd expect the Saints to be doing.
I'd like some DLC follow up, but hell, it's better than ME3's before the director's cut.
Sorry but all of that doesn't interest me at all, I'd rather have the SR world back.
The only thing that keeps me somewhat optimistic is the strong evidence that the entire game is a simulation.