This wasn’t a natural push to a monopoly though. Visa used their size and leverage to force end users to use their debit network and squeeze out the competition. They’d charge higher fees on companies who didn’t use their services. So I disagree, this isn’t a natural monopoly.
We’re talking about what can be proven, and the DoJ seems to think they can prove Visa drove the market this way. You’re talking about a hypothetical market where Visa didn’t do what they did, which is irrelevant.