casting tumblr users of 2021 as zealots is hilariously out of touch
it's a plenty vibrant platform - all of the other social media websites are way worse, content-wise. Create a tumblr account & follow tags/accounts related to your interests. You'll get a lot of quality content!
For example, there's a reason the witcher season 2's media campaign included an AMA on tumblr with one of the stars. That's where the fandom thrives.
it just isn't an especially monetizable bunch of communities. Fandom in general is hard to monetize (look at the donation-driven AO3 for another community-hub more vibrant than the ones twitter and facebook can manage.)