It’s true that “thing but not” sites tend to have issues with getting the unwanted population of the original “thing” website. And of course, some of that “unwanted” population is actually not necessarily bad, but a lot of it certainly is. I think the main way you can combat this is by disincentivizing users to bring their unwanted behaviors to the new site while incentivizing existing users to check it out. Obviously network effects dominate, but that hasn’t stopped there from being small niches where Mastodon instances and Matrix chats are good and healthy. OTOH, the point of sites like Voat and Gab were to support users who were not welcome on Reddit and Twitter, and therefore it quickly became a problem. But I don’t like the notion that you can’t directly compete with established sites - I feel like back in the day this is Exactly what happened with news aggregators. Digg and Reddit were once fairly similar sites; although Digg always had a nicer interface, they served an extremely similar purpose at the end of the day, and yet it was possible for them to compete. I think it’s harder now, but probably still doable if you can be novel enough, or if the big site becomes too annoying (Reddit seems to really be trying at this, to be honest; just try using their website on a mobile browser!)