I think you could think of it this way. If it cost $1B to attack/cause huge trouble to a country like say Slovenia with no repercussion, no country in the world would do it, because it's not worth it. But now if I tell you if you spend $1B, and you could attack/cause huge trouble and loss for the United States of America (and still with no repercussion), I would bet at least several dozen countries in this world will do it in a heart beat. My point is, as you become large and powerful, then the cost to attack you MUST also rise, it can not be cheap. Otherwise, someone WILL attack Bitcoin, just so he could say I did it, and if Bitcoin does get that big, I think the existing established banks/creditcard/governments have PLENTY of incentive to do just that.

Now the advantage of a PoS system, is that it's naturally resistant against a 51% attack. To attack a PoS eco-system, you must have resources several times the value of the eco-system, otherwise, you WILL fail. On the other hand, to attack a PoW system, you just have to buy enough hardware to overwhelm the current miners, which currently cost about only 10% of Bitcoin eco-system value. This is why there has been exact ZERO successful 51% attack on any PoS alt coin, while plenty of PoW alt coin has been attacked to death.