Mastodon isn't a great replacement for #tumblr. It's far more ephemeral, it's terrible for browsing historical posts, the media uploading capability is just barely functional, and discovery is non-existent.
Tumblr was for blogging, not microblogging. The UX was awful and the community worked around it, but it did some things the community wanted particularly well.
They'll jump to another corporate platform because nothing in the #Fediverse is both a good fit and visible enough to draw them.
@deadsuperhero I'm not sure anybody is working on an ActivityPub platform with multimedia-focused ease-of-use (both upload and scroll) as a primary goal. But the projects are popping up like mushrooms, so maybe somebody is.
I'm working with @gdorn on our (currently much lower-level) project with the primary goal of making a federated tumblr, but it's a LONG way off right now.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!