Given the number of HN users, that may work now but invite-only communities really only reenforce a community. It's difficult for outsiders to get in.

One such example is https://lobste.rs/ -- An invite-only version of HN. It currently suffers from not having enough discourse. One reason may be the small community size.

Don't we all know bad players though who misbehave every so often, so even invite only would work for so long (and that assumes there would be enough interesting content in the first place to keep people coming back.