Or what would happen if improved cryptography comes along, how would the protocol deal with this? Would a layer on top of the current protocol be enough to patch it up or will Bitcoin die?
It is very unlikely that it will be "cracked" as there are so many possibilities of what a private key can be.
Even in the event of a "crack" in the cryptography were to be discovered it could be fixed via a hard fork. Any "crack" would also likely to be very expensive to exploit.