Why wouldn't a mixer steal from a thief? To keep their credibility? To continue to get business? If you're gonna get a 3% cut to launder a lot of BTC, are you gonna just jack the entire first 100BTC order? Or let it pass and keep your 3BTC and hope for repeat business?
Of course you wouldn't send all the BTC at once through the mixer in one transaction.
That's why a mixer wouldn't steal from a small-fry thief. But a chance at 6% of all bitcoins? Every man has his price, and mine is far lower than that. I'd sooner take even a fraction of that then continue a 3% cut of day-to-day transactions.
To avoid the mixer stealing from him, the thief could split the stash and send it to many different mixers, but he would be taking the chance that the mixers are not actually run by the same person, and that the people running the mixers do not communicate with each other. If I went through the hassle to set up a mixer, I would be sure to do it several times over just in case this very thing ever happened to me.
What would I do with the tainted bitcoins after stealing them from the thief? Probably just sit on them. Sit on them on the off-chance that I ever became desperate enough to attempt to use them.
Where do you get "6% of all bitcoins"? Why would the thief send them all at once? If I was trying to launder a ton of money, the first thing I would do is not hand the entire sum to other parties.
So, bit by bit, the "thief" sends 100 BTC through a mixer. A mixer only has the opportunity to steal 100 BTC and lose reputation/opportunity at any given time. There's no way to get "6% of all bitcoins".
It doesn't matter if one person owns all mixers. The thief doesn't have to send all the money at once. Send 100BTC a day. Maybe on 4 different providers. Max exposure: 1200BTC (assuming 100 BTC on 4 mixers, 3 days per mixer). And in a year, 100K BTC is laundered.
(This is assuming mixers actually work, which I'm skeptical of, especially for large amounts.)
Yes supposedly that's the stolen amount. I'm asking how you go from that to a mixer getting access to the 6% in its entirety. One would need to be colossally impaired to take _all_ their earnings and put them anywhere at once. Personally I'd keep 90% or more in an offline wallet stored in many places. Then slowly launder a bit of it at a time...
Of course you wouldn't send all the BTC at once through the mixer in one transaction.