You understood OP wrong. He is not saying that PoW is the only viable method of securing the chain, but rather, that you need a mining incentive.
There is such a mining incentive in proof-of-stake, block signers get rewarded.
There is such a mining incentive in proof-of-stake, block signers get rewarded.
> There is such a mining incentive in proof-of-stake, block signers get rewarded.
Not in all PoS coins, actually. And arguably you don't need it. Your stake's value is contingent on fulfilling the implicit promise of the network: honest, accurate, fast transactions.