That's the thing: Apple's exercising market power does bring all sorts of benefits to users. Obvious examples include the new privacy rules they're imposing, in-app purchases allowing any subscriptions to be cancelled with zero hassle. If Apple allowed other app stores, Facebook would almost certainly move to one that let them produce a far more invasive app. And the vast majority of the users will just download Facebook from the new store, because most people (myself included, tbh) value convenience over any absolutist stances about the software they use—see the past several decades of the free software movement.

The net result: Apple losing their monopoly means a huge influx of user-hostile behavior in apps.

Now, you're not wrong either—Apple does, in some cases, abuse their monopoly, and probably takes more of a cut than is fair from IAPs. (Although, quite frankly, I'd be shocked if the money from cheaper IAPs went to reducing prices and not increasing profits.) So yes, Apple's behavior here is harmful in some ways, but it does good in others. Like pretty much everything in this world, it ain't black and white.