That's what people in the Bitcoin community are hoping. The problem is bigger than you think. What's preventing the banks from turning on a 300 PHash farm right now? Nothing.
There are people to which Bitcoin is not a thread. Spending money to eliminate a thread is rational.
for 300peta you need like 100M(1/33 of the entire market value of bitcoin, right now) dollars to buy all the miners, is this worth it? i don't think so, because even if you do a double spend, you will not be able to recover such amount and plus making money out of it, because every exchange will lock any account if they see something suspicious...
at best you could recoup 100k bitcoin(and you need 425k+ at least) from an attack, not to mention you can't dump 100k bitcoin all at the highest price, and if you dump them slowly, they will caught you early, double spend transactions are revealed in the blockchain
you will end up losing