Most "nerd" interests like SciFi have been neutered if not completely bent out of shape while going mainstream

So no, it's not nerd culture that is dying, it's mainstream "nerdism" that committed suicide.

Nerd culture was never mainstream, MCU wasn't nerd culture becoming mainstream, it was "squeeze the lemon and hope it sticks".

And it did for a while, but not understanding what's the appeal of nerd culture for nerds, they went back to producing blockbusters that kinda resembles marvel if you squint hard enough, but it's actually the same thing of watching independence day set in a World where they replaced Jeff Goldblum with a guy dressed like a superhero.

Nerd are still enjoying their nerdy stuff, while completely ignoring mainstream travesties from multi-billion-dollars corporations.

MCU is not the same MCU nerds were crazy about when they were kids, in the same way kids today consider what we called nerd stuff back then as "old people garbage"