There is a difference between pollyannaish rainbows and unicorns and optimistic/futuristic the Legion's future was never perfect after the Silver Age but it wasn't bleak. The Legion doesn't work in a bleak setting. I loved the writing on 5YL but it wasn't the Legion. The Legion lost it's foundation when it lost it's connection to Clark and none of the reboots fixed that in fact the multiple reboots made it worse is the Legion's problem not the utopian future. If someone wants to tell a bleak story in the future fine, it just doesn't have to be the Legion, a Justice League 4000 or heck Kamandi era storytelling.