Atleast having the option to communicate with threads is enough for the people on mastodon to stay on mastodon and have the choice to do so. Blocking them off will just cause mastodon users to have to make a seperate account just to merely communicate with their friends on Threads. Defeating the purpose of the fediverse entirely and going back to square one.
It doesn’t defeat the purpose to prevent a known-hostile actor from interacting with everyone on Fedi.
It’s not just your friends, it’s Facebook, with algorithms specifically designed to manipulate you and the communities you are part of - including your friends - and by engaging with them you end up locked back into FB’s reach and make it easier for them to EEE or EEC us or do even worse >.<
If you want to talk to your friends, use another app (including threads if you really feel that much need to interact with people who are only on there), or post links promoting Fedi on other platforms, to your friends.
Defederation exists to protect our network from groups like Meta/FB. It doesn’t defeat the purpose of federation to choose who to federate with.