The cryptocurrencies I consider viable for this kind of thing (mostly Monero) are actually better for low-value transactions than conventional card payments, since they have a low, fixed transaction cost (well under 0.1USD, try that with VISA/MasterCard). I'm not sure about scaling volume, but the current situation looks promising.

Friction is a problem, but I don't see why sending Monero has to be any harder than with conventional payment methods. The hard part is overcoming the network effect of payment cards.