Subs are not some abstract things where you just slap a new mod on and it's all fine. You may be abke to do this in extreme cases but there is no way this works for thousands of subs
I think the real thing that Reddit has to worry about is that upset activists could directly start spamming the site in order to justify their warnings about what will happen without mods. "Nice subreddits you have here, would hate to see anything happen to them if you don't accede to our demands."
Right now, admins will probably replace a few dozens of mods, and then the rest of the site will be back in 2 days, and nothing will change.
For example, /r/askhistorians is going private for 48h and reopens readonly indefinitely after. /r/pathofexile as far as I understand won't be going private but goes readonly once again indefinitely. etc
Wow, that'll really show them who's boss! /s