Limit credit card fees by law to some amount that easily covers the actual transaction costs and then some, but does not leave endless room for crazy stuff like

Limit credit card fees by law to some amount that easily covers the actual transaction costs and then some, but does not leave endless room for crazy stuff like kickbacks that effectively eat up 80% of the fees and thus clearly expose that the real cost of transactions can't be more than 10-20% of the fees. There you go - Problem solved! No more "tax on the poor", but cheaper prices for everyone. There's even been a model for that for over five years in the European regulation regarding this topic.