How do you fix the past?
If someone had 51% of a Proof Of Waste coin's hash power, what can you do to prevent him forking the chain?
The same way as with Proof of Shit coins. So why are we discussing this aspect?
DPoS
We are discussing PoS versus PoW here, and why people support PoS specifically.
You would need to buy up 51% of the currency supply to use this attack vector.
Just as it happens for many coins an exchange owns more than 51% of the supply. They paid NOTHING to obtain it. You are entrusting them directly (or indirectly when they get "hacked") to not fork your coin.
In the NXT hack at BTER, BTER only had like 5% of NXT's total supply - so there's already empirical evidence proving that no exchange has 51% of NXT and I assume this to be likewise true for other PoS coins.