If I want to write an app for the iPhone, I should have the choice to put it in Apple's store, some other store, or sell/give it away directly from my own web site.
Good. It IS a monopoly.
As I've said repeatedly, it's not Apple's iPhone, it's MY iPhone. I should have the choice to install apps from whatever source I desire, and I shouldn't have to go through ridiculous machinations with Xcode every week to do it.
It's both Apple's phone and your phone. And there is quite the legal / judicial precedence to back that up. Control of hardware, systems, and platforms was established with Atari vs Activision (back in the 1980's). Atari wanted to argue that it was their system, their hardware, and they challenged the straight up legitimacy of 3rd party development. Atari flat out lost. And instead though, their lawsuit against Activision, established the legality and legitimacy of 3rd party developers straight up. From a legal standpoint the case established that anyone (i.e. Activision) could write software for any system as long as there wasn't anything actually blocking it. That is if you 1) Either could figure out how the hardware worked on your own or 2) Had previous knowledge of how to write software for it (e.g. were former Atari employees, i.e. Activision) - your work and business was legal. That is, the former Atari employees who left and started their own business writing software for the Atari VCS were not stealing or using company secrets. Moreover, there was flat out no action or legal recourse available for Atari to prevent anyone from writing software for the Atari VCS. This is also what lead to the flood of software to the system and crashed the video game market in 1983.
Starting with the Nintendo Entertainment System from Nintendo this court case is very specifically -*
why*- all platforms since then has incorporated some form of lockout feature - such that 3rd party development could be controlled.
While 3rd party software development is now inherently legal - platform, system, or hardware manufacturers are just as much unquestionably allowed to completely prevent or manage 3rd party development using a lockout system or to openly allow it. Specifically that is: They don't have to allow others to undercut their requirements. Google allows 3rd party "App stores" as is their prerogative. But, they can just as much change their minds about it:
Google just banned Chrome extensions being installed from 3rd Party sites
This all stated - Nintendo itself in the late 80's / early 90's very much got whacked for being anti-competitive with their licensing requirements and agreements for 3rd developers.
This isn't really about the 30%. Apple can charge whatever they want at their store. But they should NOT be allowed to have the only store.
But, this case is very much about that 30%. In the Nintendo case - it was the developers being harmed. Previous anti-trust case precedence was: "Only direct customers of products or services can seek antitrust remedies against the product manufacturers or service offerers." The question, in this case, being posed is whether consumers are being harmed by essentially complicit developers (whether willful or not) passing on costs. To your point, to your concern, the Supreme Court says the question is legitimate and worthy of discussion. The previous precedence does not apply in this case as the Supreme Court views app purchasers as direct victims, not indirect victims.
Temper your expectations though. Nothing on the merits of the plaintiff's accusation is implied. Apple's defense of their license agreements could be accepted by court that hears the case now. The case is probably going back to the district level because a decision now needs to be made on the merits of the case. Yes, Apple gets to defend itself from the plaintiffs accusation. Then Apple would get to Appeal the decision if it goes against them.