But if a collective of Mastodon Admins decided that the Fediverse doesn’t include #Tumblr, what would be the path forward?
You won’t ever get all Admins to defederate from Tumblr and Tumblr has 472 million registered users so they still have more users than the entire Fediverse currently has.
As an Admin or a smaller instance, I would certainly side with Tumblr over whatever the sad number of users this collective managed to pull together.