One day, all these bitcoins will somehow hit the market. 
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.
Indeed. Since we are in a vast market like this in crypto, we certainly do not know where and from what hand does it come from because cryptocurrencies such as Bitcoin is just being transacted and circulated across the market coming from different addresses and we do not even concern if the source is a legitimate or an illegal one. Yes, most Bitcoins are coming from illegal activities and transactions which we certainly know and care about like we do on physical money or cash because if ever that came across from a dirty stuff, then it is not our faulty anymore because currencies are normally being circulated to be used up by many people so we do not care anymore and should not worry if ever it came from somewhere illegal.