Think about it; instead of those in charge or the instances deciding who they don't want to be federated with and thus restricting content for the users, it would be better if users were able to block entire instances instead.

We'd be able to curate our own browsing experience so much better without admin/mod drama influencing the rest of us.

Edit: Alright so maybe not exactly replace defederation, but it should still be an option available to us, and in general should become the default action before defederation IMHO.