why does it have to be a dark-web market if blockchain payments are first class citizens? I suppose the general population isn't ready for decentralised payments yet or do you have another reason for such use-case generalization?
Taxes are paid in fiat. Holding non-stablecoins would add even more of a tax headache because you'd have to track capital gain/loss as you enter/exit fiat for taxes / fees / vendors that don't accept crypto
Also the fact that general adoption has been slow so far may be a sign that there is not enough obvious value added for the average person to consider using crypto over fiat.