“Substantial proportion” does not equal monopoly, and the judge ruled in this very case that Apple does not have a monopoly. So to answer the question in your first post, Windows was a monopoly (with over 90% market share), iOS is not.
“The competition allows what Apple doesn’t” isn’t a strong point in favor of your “Why isn’t Apple regulated like a monopoly” argument
No, all developers have the option to add a link to their website in their app that kicks users out to a website to process payments. Those who don’t (or for developers who do and users choose to go through Apple anyway) have the same 15/30% cut.
If users take advantage of the link, then they have to pay their own payment processor, handle their own sales taxes etc, which will take a percentage of their earnings.
Epic is apparently losing money at their 12% commission rate, which suggests that the 15% that 95% of developers pay is extremely reasonable; I’d argue it’s a good deal.