The problem here is that platform is not precise. You could say this means that Apple should just make the iPhone hardware, and software vendors should compete to create operating systems for it. There's no hard line.
Even if you argue that for example a phone and messaging app should/must be preinstalled on a phone, Apple could allow competing apps for that, and uninstalling or disabling the preinstalled one. Then it would be much harder to argue for that they are unfairly competing in the platform they provide.
Courts are used to arguing over problems where there are no hard lines, I don't think they take "there's no hard line" as an excuse to do nothing to enable competition.
It is a hard problem because the line between an OS function and an application function is very blury. Take for example the FindMy functionality. It's open to third party devices and manufacturers like Tile, but to get all the functionality the OS has, you need to use FindMy, not your own app (like Tile used to/still does). But is FindMy an OS functionality as a built in, privacy preserving, low powered tracker service) or is it an application? Or WebKit. Webkit is the only web rendering engine in iOS. Every app has equal access to WebKit, but is it part of the platform or competing in the platform? Sure, on general purpose computers, browsers are applications, but is there any reason they can't / shouldn't be an OS service? The lower level networking stacks are OS services. The higher level screen drawing stacks are OS services. Why would gluing those two things together not be an OS service?
> Then it would be much harder to argue for that they are unfairly competing in the platform they provide.
No it wouldn't, because you could argue that they should allow competing OSes on their hardware platform.
> I don't think they take "there's no hard line" as an excuse to do nothing to enable competition.
I'm not saying they should. You're not responding to what I said. They didn't need to enable apps at all. They did, and allow things like Whatsapp to compete with their phone and messaging apps. They've enabled competition. Courts don't do anything like that.
