That's an interesting idea!
Perhaps intentionally using uuids in the URL instead of slugs and improving the recommendation/search algo (e.g take into account the average post length or cited sources in the ranking) would solve this issue. Main challenge might still be that its very hard to move an existing user base if the moderator(s) blocks all posts about other communities.
Perhaps a more democratic moderation system or a system wide rule that disallows moderators from blocking posts about other (competing) subs would work?
Yeah but then you can't easily visit specific subs. When I was younger and didn't have an account, I would just go to the url to view my favorite subs, and uuid's would make it less intuitive.
One other option sites like scored.co do is they allow subs to use their own url (like their Trump sub is called patriots.win). The site admins have kind of given up on the site though so I'm not sure if you can still do it, but it seemed like a clever idea.
These are good technical suggestions. The social problem is that ragebait => more money for Reddit.