There's nothing particularly special about a one-legged Merkle tree though. It was around before Bitcoin and it wasn't widely used for much. Without proof-of-work you don't actually have a functioning distributed currency system. What is the point of a blockchain without proof-of-work? What problem does that solve in a uniquely good way? Or are you just saying that banks are so far behind in technology in general that even this (i.e. cryptographically signed transactions) would be a big improvement?