Right, and also, a good part of why credit cards have 2-3% overhead is essentially insurance against chargebacks. I'd rather pay $51 for groceries every week than $50 for groceries plus a risk of irrevocably losing tens of thousands of dollars at some point with no warning.

If cryptocurrencies think that they can provide this service for substantially cheaper than 2-3% (note that this is an additional service on top of simply making the transfer happen, so mining fees aren't the right thing to compare), that sounds awesome, but also I'd like to understand why it's possible.

Even that 2-3% is way high, in some places like Europe and Australia, interchange fees are capped at values around 0.3% and that doesn't seem to be a hindrance in getting business done.