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"
George RR Martin made a good point- that if a nerd is someone with a lot of knowledge about something others find uninteresting, we've all become nerds with the advent of the internet. It's just too easy to go down a rabbit hole not to be a nerd in regards to something.
But going down a rabbit hole in the era of internet does not necessarily mean gaining a lot of knowledge. Usually it means binge learning other people's information and conclusions, while not making any relevant experience and conclusion on your own, thus not making much knowledge yourself. And then moving to the next rabbit hole, to forget most things you have learned just now.
For me, a nerd is also someone who sticks to the hole and builds their own labyrinth to some degree.
I think it's fair to say that nerd culture does, has, and always will exist, but what nerd culture is changes with the passing sands of time. The nerds of today are not the nerds of yesteryear (read: us).
Anime in Europe was already mainstream in the 80's and 90's with Dragon Ball (late 80's/early-mid 90's), Captain Tsubasa, Candy Candy, Mazinger Z, Heidi. On video games, well, everyone played Tetris at least once.