It's very possible that our coins are already mixed with illegal coins those were marked on the addresses. You never know.
Does this worry you much?
It doesn't for me.
When we deal with cash, we do not know the source of that cash whether it came from an illegal or legal source one hand before me. I think the Bitcoin hatters are very active to plant the seeds in our head some day we will be in trouble. There will be time when we will not have these centralized exchanges with centralized fiat, there will be a time when we will not have any centralized financial system. Wait for those days.
PS: I am not encouraging those illegal Bitcoin traders by any means. I just think that nothing can stop Bitcoin.
The thing here is that, because bitcoin has accountability some people seem to think that stained bitcoin might stain the chain, but I think your argument is great, in the sense that, just because cash isn't as accountable as btc, doesn't mean we can't make the same argument for it, if we as humans really cared about people misusing our means of exchanging goods, we'd take down 90% of all US dollars because there's cocaine traces in that many bills.
If that doesn't stop the USD, believe me it won't stop bitcoin either, this accountability will instead help bring justice, more so than cash ever did anyway