I think you missed my point, which was that getting massively indebted to credit card companies at ruinous interest is something that 1) nobody outside the USA

I think you missed my point, which was that getting massively indebted to credit card companies at ruinous interest is something that 1) nobody outside the USA does (instead, people get massively indebted to normal banks, usually for somewhat lower interest) and 2) could quickly disappear inside the USA due to competition from banks or changing regulations. Thus, it would be a very stupid move for credit card companies to rely on debt interest completely and forego merchant fees, even if doing so would raise their income short-term (which I am not at all convinced it would).