Ultimately a cryptocurrency, like many things, is only worth what people are willing to pay for it. If people decide to stop using Old Etherium and switch to New Etherium that works the same and where everyone except the hacker get New Eth for their Old Eth, there is no way to stop them.
This is a good point - the implementation always matters. If the implementation changes, it doesn't matter what the original specification was, the new implementation is the new spec, if it gains mass adoption.
