Their behavior certainly goes way beyond their legal mandate, if not in the USA then for in other countries. Certainly arbitrary, within international context all but certainly illegal. Considering the impact that have had around the globe with their abuses, arguable even covert state terrorism.
However, they are essentially untouchable. Any substantial threat to these companies will have several huge economies instantly through all their persuading power at whomever may cause pose that threat. I don't see that changing as long as these companies are such a convenient tool, in a system where literally everything is ruled by capital (how conveniently).
But the system will erode itself, step by step. Until it does come to a collapse. Good luck with that day, for I hope to not live to see it happen (it won't be pretty).
Organizing a boycott like that seems almost impossible. There just isn't enough realistic competition and going back to paper checks is just not happening, especially online.
So I don’t think it’s so much “abusing a dominant position” as “being subject to so many regulations and potential fines that it’s almost always better to just not engage in some business than to take any risk”. Lost revenue from OnlyFans is peanuts compared to potential fines for “facilitating sex trafficking”.