Wouldn't an interested party just assume A and B are both guilty and given the current taste for asset forfeiture laws, require proof of the origination of the funds? At one point does it not become possible to "capture" people this way? 10k wallets? 100k? I may be too simple to understand the math here, but in the end you've got people with guns to deal with.
You can assume all you want, but then you can end up with thousands of tainted addresses that participated in tumbling. Good luck proving anything with that to the jury.