I felt this as well - ok so this person feels a bit 'assaulted' by the sudden surge on incoming people... but it seems bit silly to create open instances and then be shocked when people actually want to join them, especially when mastodon (AFAIU) makes it relatively easy to create private spaces if that's what you prefer.

As for cross posting - re posting and re-mixing other people's content has been pretty much the basis of the internet since forever. Again, if this is something you're not happy with you should better create private forums for just your friends, that's a totally fine thing to do. Much like if you want an intimate house party you only invite your friends and eject any gatecrashers who try to sneak in.

Perhaps the clue is in the 'anarchism' bit - in anarchist circles when you have a smallish and relatively close group you can manage a very positive and supportive atmosphere without rules and protocols. But as things grow you lose that, inevitably, as having more people means needing more formalized ways of handling issues. I can see how it feels like losing something but on the other hand you have to question the wisdom of an approach that is basically guaranteed to fall apart as soon as it becomes a bit popular.