> And who enforces the contract? [...] Blockchain provides satisfactory answers to all those problems.
No, it doesn't, not if the contract touches on anything outside the blockchain. At the very least this includes anything physical (physical objects or physical services) and currencies not based on that particalur blockchain.
Moreover, blockchain isn't even allowed to be a satisfactory answer to all those problems, except when you're operating in an extra-legal environment.
Because, if a court decides that a transaction was illegal and orders you to reverse it, answering, "But the blockchain. . . !" is a great way to land yourself in jail.
Which, granted, plenty of anarchists and criminals have moral or practical reasons why they would rather operate in an extra-legal environment. But here we come to the reason why skipping the "as a..." line in your user stories is such an anti-practice: Those are two very specific user profiles. Most the rest of us would like to see QuadrigaCX's clients get their money back.
In some respects this is particularly useful in a criminal setting due to the US legal systems doctrine of un-clean hands. You can't sue your drug dealer for failing to deliver as agreed, for instance. As such, the blockchain is effectively the stand-in as you can't get sued by your counterparty over illegal transactions anyways, and the judge won't order you to reverse them.
But if you are dealing drugs, and the judge says to hand over the funds, you better believe that no amount of "blockchain" will keep those funds under your control.
No, it doesn't, not if the contract touches on anything outside the blockchain. At the very least this includes anything physical (physical objects or physical services) and currencies not based on that particalur blockchain.