I don't understand how PoS changes this. Also, that blog post does not consider CPU-mined PoW which is unprofitable for miners and sustained attackers.
In PoW, if you control 51% of mining the users can defend by switching to another hash algorithm, meaning the attacker needs to re-buy a lot of new hardware. If they’ve anticipated this, and attack again with new hardware, there is not much further defence. In PoS, users can coordinate a soft fork to burn the attackers funds, each time they re-attack the chain, until they eventually run out of capital.
