That only works in a closed industry with you having the only source of cash. There are at least 5 mining companies in the industry with heavy heavy VC investment and most of those can raise more if required. You can't empty a sink if the tap is still running.
Maybe banks already commanded more than 50% of hash power, just we don't know. Most of the actors in bitcoin mining space are short sighted and they only want to make some fiat profit, so banks will take it over easily
One benefit of the bitcoin is that it can solve banks headache of endless monetary expansion: Unlike house/national debt, bitcoin can absorb any scale of fiat money inflow without a total crash, thus the bubble of bitcoin will always become bigger after each downturn, and banks need such endless bubble to make themselves rich
However, if bitcoin is going to replace the fiat money as major payment medium, then fiat money will have no use, that will not be welcomed by banks
i would tend to argue this bolded statement: its no headache.. its their privilege.
+ you are wrong thinking mining companies are short sighted. on the contrary, they know they are the next payment processing industry (cf. bitfury).
it's like trying to reach the speed of light, the faster the harder!
