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.