This is a very good point with relation to mixing services.  A lot of people talk about how they use the services for privacy reasons.  I think that's a lot of bullshit.  I would guess that a large majority of the users who utilize these services are petty criminals who are laundering small amounts of stolen funds.  That's why you see those wearing mixing service signatures constantly harassing attempted scammers who are not covering their tracks by paying a % of their stolen funds to mixing services.  That point aside, you have to assume that government agencies such as the IRS have likely already setup their own mixing services as honeypots or are monitoring mixing services to go after any large transactions they are able to trace (which is easier than you think).  I imagine at some point in the future there will be a massive sting and a lot of people are going to get caught up using these services.  That or they will shut down and run once they realize they've been compromised and take any funds they're mixing with them.