End-users don't have much agency here, network needs banks.

In Iran, they simply don't have much choice, so banks use the only network available to them. It just happens to have been built in this century, so it's not as archaic as Visa.

Here in the US, I guess only regulators could realistically create a new network and have enough power to convince banks to adopt it.

Nothing, of course, stops anyone from creating a new network. But the network effect is THE driving force here, it's not a minor factor.