Sorry if the question is silly, but if the attacker has the great majority of hash power, what's to prevent him from updating his software & repeating the attack?
Ya - that would be up to the nature of the code change in the magical hard fork.
Even with no hard fork, the difficulty quickly increases to a point where even their super hashing speed is limited to 2016 blocks in two weeks. The 2nd scenario I laid out above (attacker starts out mining 1 block per second) has almost reached that point. They would be back to 2 weeks by the next difficulty increase.
Of course, what's not covered is that even after that is restored, then the attacker would still win the lottery for most or all of the blocks going forward. So maybe the hard fork is that you can't mine if your hashrate is too far out of line with the rest of the network. "can't mine" means that such blocks are invalidated by the rest of the network.