I feel there are too many problems with just saying 'make your own sub.'

There are a handful of subs with inactive mods but people still post to. These are prime targets for brigading and griefing that only admins can clean up in that case. If you do decide to make your own sub you'll probably want to post in the abandon subs encouraging others to follow you to an active moderated sub. But what if no one else marks your post insightful and/or fun and keep posting in the original? On top of that you have to hope you can find a decent alt name. /s/atheism is going to be a first choice to sub for people looking for non theist posts. /s/atheism2 is a goofy way to fix this and /s/nonreligious or /s/secular might work but technically are slightly different in topic. And if we are ok with leaving these subs as ghost towns that'll be off putting to new members and would probably encourage older members to start creating subs with the intent to squat names.

It'll probably help to have an automatic function to either open up subs to claim if all mods are inactive for too long. Or at least have a community effort to identify inactive subs and open up elections to fill in moderator spots.