There's been talk about Embrace Extend Extinguish coming to the Fediverse, with the counter-argument that anyone who tried it would find themselves almost universally de-federated and just building their own walled garden.
I actually expect most Masto and PixelFed instances will block Tumblr pretty fast.
This is wishful thinking. People will use whatever is easiest even if it means centralization.
There are two routes we could take to fix this. One is building social pressure and a social consciousness around federation. The other is to bribe people, like how crypto social media rewards people for making popular posts.
I actually expect most Masto and PixelFed instances will block Tumblr pretty fast.