I agree with you about harassment issues, and the importance of controlling the transfer of admin-level data between instances, but for your last scenario, doesn’t blocking only apply to users who are logged in? Assuming your hypothetical tankies and Nazis were actually posting as well as blocking, it would be easy to find them just by logging out, and there are a lot of ways to get them banned or otherwise counteract their activities that don’t require someone to interact directly with them while logged in. The case you’re describing is not the kind of situation where the most important action is to argue with them. Arguing with extremists usually just validates their delusions, and encourages them to keep doing what they’re doing.