With PoS/PoI/DPoS a sybil attack can come without any notice and with potentially much cheaper costs. (No, an attacker need not have to "buy" coins to attack, They can create an exchange/bank that pays interest/dividends to corner a good chunk of coins 5-30% needed depending upon the algo, Or they can create a popular wallet with a backdoor, Or they can compromise several large bagholders computers, Or a few large holders could short and attack their own coin, ect..)