So if all it takes is 4 miners to pull together - and that's never happened - but it would take 70k individuals with Proof of Stake - wouldn't that make it much less likely to happen???
Also, ETH has already forked over a doctrinal issue - I don't doubt it could happen again, but the PoS leaders will just directly double down.
This isn't a PoS/PoW argument - more that most public blockchains are inherently winner take all systems.