I think if the government act on this problem, they can lessen that 76%. They don't have to focus on the modes of payment of the illegal things but instead, they must intercept the people and the products outside the dark web. I'm pretty sure they can't use internet if they are behind the bars.
The owner of the dark web site is anonymous, we don't know who is behind it. It's the same when we trade Bitcoin with other people we don't know who he really is. I think the government will be difficult to find them.
There are many types of the dark web such as buying and selling drugs, other illegal products, and criminal services. It is impossible for the owner of the dark web to give out their identity easily because they will deal with the law.
Could blockchain forensic companies like Ciphertrace be able to track this info with their tech?