While I've been working on DarkSend I've been thinking about this alot. You know what the problem is with a zero knowledge proof? There's a secret key that will be attacked by governments and it will eventually be cracked. That will make every transaction that ever happened in that blockchain cleartext and allow every future transaction to also be viewed as cleartext. I'm going to venture to say the government is working on quantum computers and such crypto can be solved with shor's algorithm. When they do, they're not going to tell anyone. So it's a giant honeypot.

DarkSend is immune to such attacks, all involved crypto functions could be cracked and the mixing is still done off blockchain by the masternodes. Off blockchain means that the information is lost and that's the only way to secure it, you can't do it with cryptography.