But for you is it a real problem these 500 million usd /year ?
Of course it's a problem, I'm being charged 10% tax on my Bitcoin holding, for something that is easily replaceable and useless.
We are still all early adopters. The selling pressure ensures that new users can still buy Bitcoins at reasonable prices.
Bitcoin works at any price. If Bitcoin was $5000/coin, I am not sure the network would handle the resulting transaction volume. (Which is why we are looking at a hard-fork to raise the maximum block size.)
PoW solves the initial coin distribution problem. In PoS, banks with unlimited money can easily take over the currency. With PoW, banks can not simply inflate the price in order to buy up all the currency. If they do that, miners would scramble to generate new coins with anything they can find.