It's not. All of the DNMs have switched to Monero already. Ransomware is also changing to Monero. Anyone using Bitcoin for illegal stuff at this point will be caught.
It takes something pretty extreme to get the consensus of enough miners to matter, but the hack of Mt. Gox was that extreme.
[1] https://www.coindesk.com/623m-in-bitcoin-from-2016-bitfinex-...
This is bullshit. WHM switched to Monero, not "all of the DNMs"
DNM == Dark Net Market WHM == White House Market, a large DNM