My feeling is that this may be the breakthrough moment for Mastodon.
Tumblr/Twitter/Facebook/etc are going to die the same way malls do. One decade they're a big deal, the next people's slow shift in preferences make them irrelevant. At least they won't leave rotting husks this time.
I think the big difference there is that there are game changing advantages to end users being able to send email anywhere and to access all of the web rather than what was essentially one web site.
Federating Tumblr (and Twitter with Mastodon) seems like a lateral move at best. The worst case is that it makes it harder to use and so it might even be a step backwards.
Sounds oddly familiar.