I think it's really hard to argue Visa/MC is not a duopoly. Unfortunately anti-trust enforcement in the US has been more political than anything.
Going by that reasoning a monopoly would be even better, as the card would be accepted everywhere.
The consumer pays by lack of innovation and high scheme fees.
IMO not super relevant to topic of competition, there’s 4 networks, the fact that two of them are structured differently doesn’t really have impacts on sellers & customers (other than that the two open networks are significant larger, visa & MasterCard)
Discover also does this, although there are less available. Here is an example through Comenity in partnership with True Value: https://d.comenity.net/truevaluediscover/pub/Home.xhtml They used to do something similar for a card with Wal-Mart.
Waaaaaay past any reasonable threshold for market power scrutiny.
But antitrust is currently dormant in the USA.