I get what you're saying, that people shouldn't treat the Fediverse as a secure communcations system. There is a warning about this in Mastodon itself saying sensitive info shouldn't be sent through the DM system.
But you're treating privacy as a binary all-or-nothing thing, where anything that is revealed to spies and corporations will be visible to everyone, when it isn't.
Friction matters: any lock can be picked, but locks are still useful in keeping people out of your home.