the users should have the ultimate control over the things they share
True. Things they don't want publicly visible, worldwide, forever; shouldn't be shared on the chain. Post it literally anywhere else on the internet, and you can take it down, the site owner can take it down, the alphabet bois can take it down.
This is the one place nobody can do that; and its main selling point.
Think about it like this. Imagine I post something a powerful person doesn't want known.
If I can permanently, entirely delete my posts from the chain, he only needs to shake me down for my keys then use them to do so, and his problem goes away (as do I).
Since that's impossible, I can post all sorts of stuff about all sorts of people, and they can't do anything about it. Even more; if I suddenly go missing or turn up dead, there's going to be a lot of scrutiny on my posts, which can't be deleted, even by somebody in possession of my keys.
There's no problem to solve here. This is the solution.