Zuck needs Yann LeCun and other senior researchers at Meta a lot more than they need him. If they were to quit there would be a line out the door to hand them as much money as they want to start a competing open research lab. I bet a ton of top researchers from other labs would be happy to join too, since from what I've heard from friends they're all miserable from dealing with incompetent management.
On current trajectory one of Sam Altman / Zuck / Elon will end up having full control over the frontier models that are trained on their huge new clusters. All 3 of them are unaccountable to anyone.