If you choose to defederate you have isolated your users, made a decision of whom they can and can not interact with. Again, if this was agreed upon by the user at time of sign up (like niche communities) it's understandable. There's not even a migration option on some of these apps like lemmy, you can't change your instance easily if they decide to defederate.

TBH, I haven't delve deep in exact architecture of these systems. AFAIK posts and all data remain on instance of OP but when you like, boost etc. it's not like your data is transferred to that instance, and you've lost your privacy.

Each time somebody interacts with your post by creating a reply, boosting it (retweeting), or favoriting (liking), this needs to be propagated to other servers (where your followers are located).

If the interaction itself happens on another server than where the post was originally created, first we need to notify the origin server and only then perform the propagation.

The architecture of Mastodon

But I guess you're making a different argument. Yes we're using threads every time we interact with a user there, but again we're not seeing ads, we're not giving every single click and page view to meta. It's not ideal but it's way better than using threads UI!