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.
Of course there are difficulties from the government to be able to control overall if the government itself does not facilitate and can accept bitcoin. then there will be more people who want to go beyond the rules made. there must be a role for the government so as not to multiply.