And of course miners can't switch mining pools .
EDIT: And whether it cost a billion or a few dollars to create your own mining farm that controls >50 percent of the network doesn't matter at all...
My point was that increased amount of hashrate doesn't increase the security of the network, and it doesn't. The existing pools are still as fragile as they are, no matter the hashrate.
About the cost of obtaining control over the network. PoS mining solves the same problem much more efficiently. Someone would have to buy up enough coins to obtain control, but that means that he would also have the biggest stake in that coin and the biggest interest for the network to be secure. The same problem would be answered MUCH cheaper and future-proof. The bitcoin miners are the monkeys on markets back. PoW mining is keeping the production cost high, while it shouldn't have to be high. The future of monetary services is in offering more efficient and cheaper solutions, not offering new bloated systems that reward useless people doing useless work.