iOS is not a closed system so long as third party apps are allowed. If Apple kept it closed to only first party apps as it was when the iPhone was first released then yes they could do whatever they want with their platform. The second they opened it up to third party developers to sell apps on their platform then they've created a market that can and should be subject to regulation, just as Microsoft's practices were subject to regulation in the late 90's.
Apple makes its own hardware, but unlike Microsoft or Google, Apple doesn’t offer its hardware or OS to third-party manufacturers and that makes a huge difference.
It’s not the same if someone invests billions in the development and production of their processors (A, M, S, T, W series) and the rest of the hardware lineup. They have their own operating system (iOS, iPadOS, watchOS) optimized for just that hardware and now they should what?
Offer all this for free use or by order of some government to others?
<- sarcasm
* But nooo! We want to do without investing in R & D, in hardware or in the operating system and we want to earn money from such a company.
* Oh, but Apple already offers you that (App Store).
* But, but we want people to buy apps from everywhere, not to pay Apple, Apple should not have their apps on their device. Well here is for example Microsoft or Google, etc.
-!>
But Apple is neither Microsoft nor Google, for the above reasons
Then McDonalds should offers hamburgers from other manufacturers in their restaurants, because they sell Coca-Cola.