That is ignoring that instance owners can decide for their users what instances they can and cannot engage with. Which is fine, it is their server they can manage it how they like. That is not the user deciding how to interact with other instances and communities.

I see the whole federation as a bit of a paradox. Users are allowed and encouraged to interact with other instances and communities as they please. However, if the instance a user belongs to decides to defederate from another instance. Users now have to create another account to keep interacting with that instance. Which kind of defeats the purpose of federation. Now users are forced to create an account for each instance they want to continue interacting with.