You raise very good points. And honestly, I have been thinking about them for sometime too. nostr:npub1cz2ve34nk0ukn0ph4yq2qx3ud8rfy5e0ak4epx42dn8gha0sdgpsgra9kv has brought this point too.
1. I consider all the clients to be the equivalent of browsers. Like firefox or chrome can do little to prevent people from using internet for unlawful stuff.
2. Like you said, I see a future where relay operators will ban content that's unlawful in their jurisdiction. That's not enough too.
3. I see a future where where a client will stop showing content from a relay (and may be show a warning to people writing to that relay) if it exceeds a threshold of reports about unlawful content.
4. No matter what the case is, the solution will revolve around relays, with clients creating mechanisms for assisting in reporting bad content to a relay.
In fact, the dark web illegal stuff is an unsolved problem even today.
Good news is that we have good people (in my opinion) with great outlook and technical skills in our #nostr community. We will have a solution soon 😊