A hard fork would be impractical–what do you do about all of the transactions that occur downstream? What about the people who innocently traded assets with the attacker? Once the ether they’ve stolen gets laundered and enters general circulation, it’s like counterfeit bills circulating in the economy — it’s easy to stop when it’s all in one briefcase, but once everyone’s potentially holding a counterfeit bill, you can’t really turn back the clock anymore.
A hard fork would be impractical–what do you do about all of the transactions that occur downstream? What about the people who innocently traded assets with the attacker? Once the ether they’ve stolen gets laundered and enters general circulation, it’s like counterfeit bills circulating in the economy — it’s easy to stop when it’s all in one briefcase, but once everyone’s potentially holding a counterfeit bill, you can’t really turn back the clock anymore.
Also, a tweet from Vitalik:
https://twitter.com/VitalikButerin/status/887782650026631168