I can't really disagree with your subjective experience of playing Dark Sun, and I agree with you that the original boxed set is the best way the setting has ever been presented.
However, I absolutely want to see 5e
try to duplicate the feel. 5e so far under-serves low fantasy, sword and sorcery, and sword and planet. Perhaps--in the same way that WotC is serving a new noncombat style of play in Wild Beyond the Witchlight--they could give those S&S styles and themes a try in a Dark Sun book. The ultimate outcome might not suit you (or me), but maybe it would--and the original version of Dark Sun will be around whether the setting is revisited or not.
Dark Sun is probably the most pro-environmentalist secondary world that has ever been conceived (if anyone can think a better one I'd love to be corrected here), and it's anti-racist in that it critically depicts prejudice and has genocide in its deep backstory. But the other themes you listed strike me as a bit curious.
Dark Sun is anti-authoritarian, I think, with all its petty despots and oppressive social structures.
But is it anti-fascist? The sorcerer kings exercise state power with no ideology--or with a religious one--and all their societies are very traditional. The pervasive cruelty feels
Biblical Egyptian or Assyrian, not modern. I realize there are
many definitions of
fascism, and that Dark Sun's prevailing societies might fit some of them, but this seems incidental to me. What makes it strike you that way?
And is it anti-imperialist? There aren't any empires in Dark Sun, just city states, and they don't control large or diverse territories like, say, the Athenian empire did. There aren't client kings that carry out the sorcerer kings' wishes or mercantilist trade dependencies of raw goods for finished goods. It's just old-school
stationary bandits collecting taxes with the help of an oppressive professional bureaucracy. Even the genocidal wars of the deep backstory come across more as
movements than as imperial conquests, like decentralized 15th century religious strife or 19th century nationalist uprisings. Where does the anti-imperialism come in?
...Just a bit of friendly pushback--I agree with your point in general.