Does it mean back to cash carried in bags and placed in vaults with no online security?
we would have more to worry about than btc.
On a honest note, if you come to think of it, hackers or the bad guys per say out there aren't really looking for the fall of bitcoin rather what they are looking for is ways to steal and accumulate Bitcoins as much as they can. I bet you 9 out of 10 hackers will pick stealing bitcoins over crumbling the network , it's basically the same reason even when the 51% rule exists, literally almost no one is aiming to use it against the network rather they want to use that hashrate as an opportunity to make far more profit instead.
Yeah well if sha-256 fails due to a badass computer one could simple square it
so sha - 65536
the square would be impossible to solve quickly
my fear would not be a fast computer my fear is a new math yet to be invented which would be more dangerous as it could over come the sha - 65536