I could definitely see that, but I don't think that those economists would be likely to go on to argue that the best way to deal with a collusive duopoly is to

I could definitely see that, but I don't think that those economists would be likely to go on to argue that the best way to deal with a collusive duopoly is to make it easier for the duopoly to effectively ban competitors from the market by blocking access to their payment terminals. If you think that a company is a monopoly/duopoly, that's an argument for forcing them to support more interoperability with competitors, not less.