Expiring/disappearing public messages on AUTH relays are the most useful for protecting most people's privacy, whether the message itself is encrypted or not.
That is because the biggest privacy problem most people (and especially **WOMEN!!!**) have is people digging up some shit they drunk-posted 9 months ago and sending it to their boss, or their ex Internet stalking their old posts to figure out where they live, so that they can poison their cat or climb in their window at night.
No sane woman likes the idea that every damn chat she has is forever and archived on 5000 relays, all over the globe. And it being a private group doesn't help as much as the event simply getting hard-deleted in a timely manner because the person who can cause them the most trouble might be in that group.
The nation state is not the primary enemy, it's the apex enemy. The primary enemy is closer to hand.
