Here's the scenario I imagine:
A state actor deploys a quantum computer to de anonymize TOR, I2P, Freenet, (etc)...Arrests go down on a scale never before seen. buyers' mailboxes start to empty out until a market emerges that is based on post-quantum cryptography...
Thoughts?
I have unfortuantely come to the exact same conclusion you have.
Hopefully they either won't be too interested in doing so or we will have post-quantum sufficient encryption before the darknet is entirely destroyed.