They need to stay up but use their subreddits to publicly plan Reddit "exits" for each community. Subreddits should start polls on where migrate to (discord, lemmy, mastodon, etc etc), and then start systematically cross posting anything posted to their subreddit to the new space, with comments turned off on the Reddit version.
The key in a negotiation is to have a crystal clear, realistic picture of the other party's constraints, motivations and ethical framework. In this case, it's clear Huffman has hard constraints around making Reddit profitable. He could easily do it by reducing headcount but that itself would defeat the purpose because it would devalue the IPO (who wants to buy a shrinking company?). He clearly has no moral or ethical compulsion here. So there isn't another option than to threaten what really matters to him: it has to be made real for Huffman that he's going to lose this if he doesn't budge. The only thing that will convince him is actually seeing users permanently leave.