You don’t have privacy in public, that’s the difference between public and private. If meta wanted to they could scrape the entirety of the fediverse every weekend without anyone being federated with an instance of theirs. So that isnt a good reason for defederating.

I literally said that: "There’s this weird argument that our data is public so privacy violation is okay."

It doesn't matter if it's public, they use their server for data mining and we can prevent that. Someone may collect our data secretely but it doesn't make more open approach to data mining any better or more ethical.

given that it can be done at any point I dont see any reason to do it pre-emptively rather than take a wait and see approach.

The longer users on your instance interact with content on Threads, the harder it will be to defederate (people will get angy about losing their content and their reach).