I think you over-estimate the usefulness of BTC in darknet marketplaces in this moment.  Following the takedown of the largest marketplace, ALphabay, and subsequent sting operation against its short-lived successor, Hansa, the non-anonymous nature of BTC has been seriously questioned by those concerned with op-sec.    Just google Chainanalysis to see how the open ledger is being forensically examined.

Furthermore, the amount of darknet trading is still waaaaay down on what it used to be, as vendors and buyers let the dust settle before leaping back in.   Any new marketplaces that spring up will almost certainly encourage use of anon-coins like Monero, Z-Cash, etc , as these offer far more protection than BTC.

Im not saying that BTC wont still beused for nefarious activities, just that those doing so are idiots.