Like with Star Trek, it's the optimism that I admire in the Legion. If people think that science fiction has to be always pessimistic then they're forcing a sameness and predictability on all sci-fi. If so much is dark and depressing, the bold choice is to go in the other direction, not try to copy everything else.
And why do we think that it makes more sense for the far future to be so dystopian? The forces in the world right now that would create such a dark society are the ones that deny things like climate change and use up resources in the present without preparation for the future--they are so screwed up that they can't save themselves and they will destroy this planet in fifty or sixty years--nobody is getting out of this century alive and there's never going to be a 22nd century. Yet if you're proposing that human beings will be alive a thousand years from now, that's a huge leap of faith and an incredibly positive message. It means that reason and good judgement won out in the end and those self-interested idiots didn't survive. Hurray for evolution.
What doesn't ring true for me is the science fiction that implies we somehow overcame all our bad impulses and reached out to the stars and survived to do great things, yet insists you must regard that with a jaundiced eye and look at all people as garbage and adopt a misanthropic view of everyone. That's just silly.
As far as the look and feel of the 30th/31st century goes, as I said before, I play a game with myself and I try to imagine what is really going on. You can think creatively and imagine that this is all a constructed reality and there are more levels to the future than we see at first. After all, it's the future. It's easy to use science fiction to explain how the world can appear one way in those stories, but in fact this is not the whole story--it's just a superficial image.
Personally, I doubt that if we survive that far into the future that we will be living in a material reality. Already the push is to make people acquire stuff that isn't manufactured in reality and is only constructed in an imagined reality. So I think we will be living more and more in this imagined reality. And if you're living all your life in a fake world, then that world can look however you want it to be--you don't need mateiral objects in your fake world, so there's no need for heavy industry. In fact, you could either be living on a planet that's a garbage pit or a pristine natural preserve--but either way you don't see it, because you're living all the time in your own constructed immaterial world.