Fortunately, that's not actually true. If the community agrees when the last valid block is, all the transactions before that will be accepted as trustworthy. The DHT (now moving to freenet) ensures that this transaction history is reliable up to the date of the exploit. Then it's just a social problem of converting into a less flawed system.

But social problems tend not to end up as near the worst case as is often feared. What you're describing is like saying that if we break the crypto underlying git repositories, you can trick the world into thinking you are the original author of linux. Obviously, no one would fall for that. But every git commit after the exploit, or from sources lacking a large consensus, would be of suspect authorship. It's a big problem, but not exactly the way you imply.