The only thing keeping users in line would be the fear of losing their “brand”
This is solving a non-problem. Yeah stupid script kiddies and trolls might care but that is noise easily blocked. The actual people causing harm, committing massive crimes that flood the system with government, or causing massive DoS attacks, none of them care or even want to have something they could lose on a system like this. Its better to be anonymous and not have a brand.
Look at what happens here in the Fediverse. People take time to exclude the havens of the problematic and that resolves enough of the issue to make the services work. But that means that someone is making decisions, and that someone can be targeted to take down a site or to not defederate even when the community thinks its best. There is still a human involved that can be bought or beaten.
The only way to make a system where people follow the rules is to make a place where people dont care to break them. Rules give those who follow them justification for punishing those who don’t. They don’t actually stop people from breaking them.
I think if this system can be hardened against attacks and its easy to deal with spam then we all just coexist with the shit that happens in the background we don’t see.