I don't see the problems with an optimistic future. Obviously dangers and threats still exist at that time and the Legion would need to save the day. That's not incompatible with flawed characters who have their own personal issues that need to be worked out. It's also not incompatible with stories that don't have clean endings with all loose ends ties up. With an optimistic future the heroes need to win. They can occasionally fail, but generally, they need to succeed. A pessimistic future means that they don't win the big war, but may succeed in small battles.

If we want true diversity in comics, shouldn't we have a variety of settings and ideas? Doesn't the idea of "dark is in, everything has got to be dark" limit what series can offer? I don't see anything wrong with the Legion having nice things in a nice place. One can so easily find a future that's yucky.