The blockchain holds the coins in escrow. They are not held by an individual. If the chain is extended then they are returned with a reward to the original owner. If the chain is not extended, then the record of them having been escrowed is superceded. In this case, the client would judge that the coins had never been escrowed and they would be returned to the original owner without a reward. These are the only two possibilities. As in the current system, clients will only recognize valid chains and miners will only extend valid chains because of self-interest.
Collusion. People can collude to extend invalid chains. The same arguments applies 100% to the proof of work system. Any group can collude to manipulate the blockchain. They don't do this because it's too expensive to be worthwhile. That's the point of proof-of-work. It forces attackers to spend resources to get their way. Proof-of-stake does the same thing. Control of the blockchain would require accumulation of a majority or near majority of all extant coins. This can be done, but it would be expensive.
Would you really want to spend $10 million to acquire a bunch of bitcoin, fuck bitcoin up, and then try to sell your $10 million of bitcoin back to people? No. For the same reason you wouldn't want to convert a warehouse full of graphics cards and a power plant worth of electricity into bitcoin and fuck up bitcoin. You don't throw a large sum of money into bitcoin in order to make bitcoin worthless. It is a money losing proposition. If you are willing to lose large amounts of money to screw bitcoin, then neither of the two systems can stop you.
Please, think carefully before asking questions. You are inventing distinctions between proof-of-stake and proof-of-work. If you have a criticism of proof-of-stake, ask yourself whether it applies in the equal measure to proof-of-work. If it does, then there is a problem with your reasoning.
I'm sorry, but this is going to become tiresome very quickly. From now on, I will only respond to critics who employ structured, logical arguments.