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.