But not specific customers. Affluent consumers often use credit in advantageous ways. That behavior is subsidized by consumers of more limited means. I get 5% cashback every time I shop at amazon, 6% at every grocery store, 5% on every tank of gas and 2.5% on every restaurant, etc. (Sallie Mae Rewards Card, Amex Blue Preferred, Sallie Mae again, and Chase Sapphire, respectively). Access to credit most certainly costs me "negative money."

The only way what you say is universally true is if you hold that all costs of doing business are passed along to the customer "one way or another" which is sort of true but not a really useful way of thinking about it.

All payment options, including cash, have associated costs.

It's certainly within the realm of possibility that the credit card networks, as a whole, are more efficient than any next best alternative, and indeed likely that they are so for certain specific actors within the marketplace.