I expect the main cost of servers are the hosting of subs, not maintaining users.

Which is easy to avoid, if you want. You can disallow the creation of communities on your instance.

So users can still join and make it their home, but they will have to subscribe to communities on other instances.

I’m not sure about the ethical implications. It means you’re sort of leeching off moderation and maintenance from other admins.

I guess it’s a negligible point and totally fine, but not sure. Would be interesting if community-admins could share their thoughts on this.