Even if everything was one system, how do you handle side effects? An order I make on Seamless causes physical inventory to be consumed. That's just de nature of the system. No way to undo that. So there's still a reason for account takeover there, if someone else could consume on my dime by hacking my account temporarily.

> In the future, almost every action user might do on an information system ought to be rolled back cleanly and easily once presented with higher priority key

This won't work well unless the world contains just the one information system. That scenario has other problems.

Regarding the bitcoin: you can't take over the blockchain while thousands of clients around the world have the "common sense" to not accept invalid blocks.

This means rogue clique might roll back a few transactions, but they can't steal money from static accounts or create coins out of thin air.

You can't roll back information leakage to another system.