> I don't, for a minute, think that the 30% would not be paid by the end-user if Evil Corp A was not in play. B and C would just raise prices.

That's what I hint at with "basic economic theory". Evil Corp X wants maximal returns and there is a price point at which they get maximal returns. They can't just raise prices infinitely, because nobody will buy the service. If a middleman takes a cut then that optimal price point moves. Explaining it as the cut being shifted onto the consumer is too simplistic of a model. A part of the cut is shifted onto the consumer and the rest is lost by Evil Corp B, to maximise their total revenue.