Already this might have discussed. The growth of bitcoin as well other digital currencies keeps progressing with time. By the time common people too have begun to use cryptocurrencies on their daily life. Compared to the fiat usage, common people using cryptocurrencies were low. When the usage increases what will be the preference of common people. Whether it gets used as investment, gambling asset, trading asset, alternate payment, darkweb usage source.
I'd say alternate payment, but most of them are increasing at the same rate. Perhaps its simply a matter of people simply finding out about it.
The "dark" web and the normal web are about the same thing, one is more anonymous than the other, but that's it. Bitcoin is probably unsuitable there unless its perfectly legit activities (such as donating for a cause, or whatever).
Thing is, and this i have noticed, people with "good" fiat see little use for an alternative means of payment, because their fiat is so strong and handy, they think their banknotes and coins will retain their purchasing power forever, and because it only loses it about 2% yearly, it goes unnoticed to them.
While in my country we cannot live without digital payment, mostly in the form of debit card, simply because there are not enough banknotes around (you can only withdraw one (1) 50k banknote per day, equivalent to about 3 USD currently (currently the largest denomination, made a couple of months ago).