Sure, it is kind of decentralized, but the admins of that 30k instance are effectively able to read the DMs, impersonate users and delete their content.

There's a working group on end to end encryption already and I do believe they will solve that problem.

Archiving content is trivial and can be automated. It's also pretty easy to migrate between instances- I started on one run by a friend but which I felt was too small, moved to one of the biggest ones, and then ended up on hachyderm. I may end up moving again as I feel like the service is getting rather big- it's one of the largest instances now and there are benefits to being on smaller instances that tend to push people towards them.