I kind of foresee this ending with rooms getting spammed with posts that break the rules since the people doing it don't have any sort of barrier of reentry.
I understand the intention but it's important to be realistic about what the actual outcomes are. Is there anything stopping extremists from basically just flooding all rooms with hate?
The network effect is one reason, if users are responsible for the content then there will be a shortage of content until enough users join.
Beyond that, humans are social creatures and want to fit in. If your community mainly hangs out on one platform, people will want to be a part of that platform. What helps is if people with a large enough following or entire communities make the decision to change platform, but that can be tricky.