Once tumblr has ActivityPub integration it'll have the largest node in the network. Is it still Mastodon? Isn't that just tumblr and the AP integration is an alternative RSS/Atom? Then Mastodon becomes Google Reader with comments and stars?
In my eyes, if a bunch of different platforms have a reasonable degree of interoperability, and Mastodon is connected into that datastream, then Mastodon has won (and not doomed).
Because in the end, the federation allows people to take their toys and go home if they so choose, and the risk of being censured or "de-platformed" essentially goes away.
I think about email a lot in these conversations, and I remember when we used to have to set up sendmail and all sorts of other crap to have email. Now, sendmail has basically gone away, but we use email more than ever, and billions of people are still connected. Sendmail was that original structure that that broke apart the proprietary & enclosed email systems like CompuServe, Prodigy, AOL, etc.
To me, there are still issues with the email ecosystem today, but I don't lament sendmail's loss of market share, and I don't think of it as "doomed" but rather a huge success that spawned a massive industry. I think of AltaVista as doomed, because Google replaced it, and there was no shared or interoperable search ecosystem.
Does that make sense? Hard for me to put into writing without rambling.
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EDIT: meant to add that I liked your article, and I could tell you put a ton of work into it.