That contention simply isn't reasonable. Apple was charging exactly the same right form the start of the store, when a lot of pundits and industry analysts were still saying the iPhone was a dead end. You can't accuse them of abusing their power as an industry whale to charge more, when they are charging exactly the same as they were when they were a minnow, and are still charging pretty much the same as every other comparable business. That just makes no sense at all.
That doesn’t mean it’s settled, or that the opposite argument isn’t also reasonable, but it’s certainly a valid controversy.
That doesn’t mean the now National chain isn’t doing other things that are anti-competitive, or that their other business practices must be ok, but clearly the market has decided the shake is worth $5.
I would say the fact that the case survived a set of initial hearings before a Federal judge and hasn't been dismissed means that it meets a very clear and precise definition of being valid and reasonable in a legal sense.
Steam, which operates on completely open desktop platforms charges 30%. Even Humble Bundle charges 25%, operates on open platforms and the only thing they do is marketing. Another 5% for running a store client and associated services hardly seems excessive. So nether arguing that open platforms would push down prices, or that app store competition would push them down has actually happened in the real world. 30% is very much validated as a competitive price. If the big stores did cut prices, this would arguably squeeze out the smaller stores and operators like Humble Bundle, reducing choice even more.
That 30% is charged to developers, not consumers so in this case it's developers that are the market.
Apple and Google will both argue that it's not a monopoly because they're competing with each other. Not sure if they can in any way argue that it's not a duopoly however.