Bitcoin is already becoming quite unattractive for the criminals, as a lot of them are getting caught while converting the "dirty" coins back to fiat cash. For a while, anonymous coins (such as Monero, Zcash and Deep Onion) were being talked about as the preferred payment method in the dark web, but these coins never made it due to their low liquidity. I guess the criminals will soon go back to their old modes of payment (i.e those using fiat cash). Physical cash remains the most anonymous method of payment even now.
And it doesn't work on darknet markets,
which clearly aren't going away. DNM vendors need to accept bitcoin because it's what customers demand vs. privacy coins. As for fiat money,
it doesn't carry much value on the dark web.