Yep.

Either you radically gatekeep your community to ensure it only allows people discussing topics you find interesting and watch it die as newbies get turned away while stalwarts leave for whatever reason, or you welcome "new blood" with their own interests which are not always completely aligned with your own and watch the community change around you.

And it's not just forums, it happened in usenet, it happens in social circles and friendship groups, it happens in suburbs or neighbourhoods. I'm pretty sure it happened around campfires and caves 50 thousand years ago.