They cherrypicked a few out of thousands and Silk Road was a tiny tiny percentage of the drug market. What will happen when they are 10x the size? Will that type of law enforcement scale?
Also, adding levels of indirection only increases enforcement work linearly.
What you'd end up with are just a few organizations with the resources and expertise to outcompete other operators in terms of opsec and secure fulfillment, while wannabes and new operators serve as fodder for enforcement. Somewhat like today's situation.
Also, since enforcement depends on local enforcement, local corruption will increase.
There is no way they'll let digital black markets exist without creating a super-expensive heavy handed machine to attempt to stop it. Whether it is practical or not.