POS make the whole system more closed to outside people: Early adopter not only benefit on the price appreciation, but also gain in dividends. This will cause less and less people joining this system

An important part of a distributed system is to continuously give the new comers incentive to join the network, thus new comers with newer technology/better service can re-balance the whole mining picture and take over the throne. For example, deepbit's leadership were replaced by BTCguild, then Ghash.IO and discuss fish, etc... This will make sure the system is enough decentralized and there is always healthy competition to make the network strong and fresh. Miners seek profit, but also have the voting rights, this is a very important aspect

But in POS system miners would lose the incentive due to losing of voting rights. What they do is just make the early adopters rich, and if there is a situation need a protocol level change, they don't have voting rights, they will abandon this closed system

In PoW system, the decision making rights is always stay with the current most actively working miners, that is the best way to distribute the decision making power, although there is certain risk of 51% attack. But the 51% attack is not a big problem, because ultimately is the majority of bitcoin miners decide which chain they select to work on, not existing bitcoin holders, this is a key difference (You can buy lots of coins using printed fiat money thus gaining more stake, but you can't buy a farm with huge amount of hashing power since those guys who are capable of making hashing machines will inform others at the first place)