After all the man that developed bitcoin system is not a special being, he is also human being like us, and nobody can tell me that he is the only human genius that we have, if he has created bitcoin system, they should also create a system that will block the activities of these dark web criminals, I believe it is something that a guru can come up with to always trace them.
I think that Bitcoin is so linked to darknet now that a Bitcoin with zero value will not come till another protected and anonymous currency appears. Bitcoin will be big for sure, illegal activities are also present in FIAT but everyone live with FIAT so the percent of illegal use is less.