[once_inc]

A simple fork invalidating coins originating from the double spend would be enough. You've just spent $2B to achieve something that the network can invalidate with a dozen lines of code. It would set a bad precedent, but it did cost you a lot of money.

[wellboy]

So, you can undo any 51% attack with just a quick hardfork with a few lines of code?

[jhall1468]

One hell of a straw man. Bitcoin isn't going to do that. It isn't Ethereum.