POS is fundamentally flawed. It increases centralization because people won't want to validate themselves and instead delegate their stake to "trusted institutions". Essentially, you're recreating the existing financial system. POS is also less secure since you only lose X dollars for misdeeds but have the power to do far more than X dollars of damage. And if all you want to do is damage, that slashed stake is just the cost of doing business. And if people are staking with you, you don't even risk losing your own money. And although the "guarantee" that the validator is doing right comes form the slashing of the stake, the enforcement of that slashing is political (i.e. others have to gang up on you to take your stake). When enforcement is political, wrongful slashing is inevitable. Finally, when a 51% attack is possible with validators, it's imposible to undo it...unlike POW which can call on the community to start a few miners on their PCs.