No one cares about numbers like that. That's way, way below the convenience threshold for a typical consumer. Most of us, me included, would rather pay an extra 3.5% than fiddle with deciding on which card to use.
Which is to say: the market has spoken. We've settled on the duopoly not because it's a trap but because it actually maximizes utility. The uniform convenience of "credit cards just work" has quantifiable economic value. And it turns out to be somewhere around 2-5% of the transaction.