> How is that any harder than getting into a legally binding agreement with anyone else about anything else under the existing system?
I never said it was harder. I'm merely pointing out that the proposed solution to this particular usability issue that crypto-currencies suffer from involves reverting back to a web of mutual obligations enforced by courts of justice, which entirely defeats the purpose of using a crypto-currency.
You can make sure they don't have enough of the keys to move your funds without your consent. Making a legally binding agreement that they have to give you access to the key they have if/when you demand it is no more complex a legal task than any other instance where you trust someone to custody something for you.