If you discovered a significant shortcut to hashing and then went back to block 1 and re-mined every block until your branch was the one with the most proof-of-work, I'd have a hard time trying to claim that your branch wasn't the legitimate one, according to Bitcoin's own rules.

I suspect in this hypothetical scenario, however, the bitcoin developers would write a new rule.

Uh... yes? Are you seriously trying to imply that would be legal?

If someone ever cracks modern encryption, that doesn't mean they can do whatever they want with everyone's accounts everywhere. If you find an exploit and exploit it, that's illegal.