There are many good books on this. But the bottom line is a 51% attack would be known to be coming LONG before it happened. And probably action would be taken to prevent it (1000's of us turn on old mining equipment and many other things).

But even if there was an attack - it's not a death blow - it's a minor ripple in the block that could be righted in the next block once the attacker and situation was known.

It's obviously more complicated than this - but the only thing the banks can do is block the on/off ramps. And if they do that they will just drive innovation on the currency side of #Bitcoin.

Nothing banks can do OTHER than get their own house in order and compete in a FREE market.