any sub can remain private, and only allow posts from a select few, these subs could simply boot everybody and only let a small core team in and post regular updates so the sub is active, then Reddit can't just give it to someone else, not without an even bigger exodus. plus you can't just get a new mod team, especially for subs like askhistorians. They're very thorough, use many tools themselves that use the API to handle moderation and aggregate posts etc...
I mean, at this point they might as well call up Elon musk and ask advice on running a social network.
You're right they could keep the sub alive, but if they don't want to moderate anymore and their blackout failed to get the change they wanted, why would they bother?
They don't need to call Elon, there are plenty of far-right radical incubator subreddits they could ask for assistance that would give them the same advice.
I mean, at this point they might as well call up Elon musk and ask advice on running a social network.