In a PoS currency how do you guarantee that a 51+% stake holder remains a benign actor?
It's only logical that the 51% group of holders will not have bad intentions against a currency that they themselves have a majority stake in.
You have no guarantee in a PoW system neither, since it only needs about 10% of the value of the eco-system to 51% attack. Also the other problem is you don't even need to own the currency to attack it, therefore the attacker have no stake in the system.
To 51% attack a PoS eco-system, you yourself must be an extremely large stake holder in the eco-system, which means you are basically attacking yourself. Not to mention you need extremely large amount of resources, at least several times the value of the eco-system, to achieve 51% in the first place.
Just plain wrong on all accounts.
51% PoS stakeholders have every reason to reverse transactions. Because they use TOR, nobody knows they even have 51%. They can attack transactions to grow their wealth. When they are rich enough, they can sell their stake. With PoW, they have to invest in equipment that is traceable so they can't attack anonymously. When they want to cash out, their equipment is obsolete and worth much less. As far as the value of the ecosystem, 51% is 51%. That means just over half, not "several times the value."
In DPOS it is not the stakeholders who generate block but delegates (comparable to pools in Bitcoin except that the DPOS delegates/pools can be voted in and out) which makes your argument invalid.