"Small" is doing a lot of work there.

Large entrenched companies have leverage small businesses do not, in the same way that a large moon orbits in a way a test particle of infinitesimal mass does not. We already recognize this with respect to monopoly law: you lose your right to do certain things to your competitors precisely when you're large enough that you could reasonably suppress them.

That is essentially what we are talking about here: a duopoly that is actively suppressing competition. My understanding is that the big-two payment processors don't just refuse to process certain payments, they also refuse to work with banks who work with payment processors who will. Assuming that I am correct in that understanding (I might not be, this is not my area of expertise), that would prevent (or at least hinders) someone from just saying "there is a market need here" and forming their own payment processor to fill that need. To me, that seems like a problem for the exact same reasons that monopolies are a problem, and regulating against monopolies is not particularly controversial.