Sha256 will be replaced, but even if it's cracked before a replacement, there are alternatives ready to be deployed with a fork. It'd be ugly but not lethal. C

Sha256 will be replaced, but even if it's cracked before a replacement, there are alternatives ready to be deployed with a fork. It'd be ugly but not lethal. Changing the hashing functions, encryption, and network verification are all built into bitcoin. It's designed to be resilient, and to use consensus when adapting over time. It'd be really stupid for people to go "oh no, someone cracked sha256 and mined a million blocks, I guess we're done with bitcoin now. Buh-bye money!" They'll fix it in the case of a noticeable exploit. If someone were clever, they'd mine blocks surreptitiously, sufficiently spaced out over time and different networks so that it looks like normal mining. They could get a few blocks per day for maybe a couple years until the upgrades. The impact to the system would be minimal.