Some clarifications for this thread.
1. Protecting the telecommunication network only applies to the use of the radio network and related software which the manufacturer will have to certify to before a carrier will allow it on the network. Apple or any Android phone manufacturer has to do this. This does not apply to the app level or the OS support for it which is like using a modem to connect to the IP network. This is easier to understand in a laptop with a built in wireless modem or a dongle. Only the wireless modem needs to be certified for protection of telecommunication networks not the software on the laptop. So, Apple is not required to certify or screen apps for that reason but it will be liable on its own if bugs in its own platform software allow an app to disrupt the network. So, this has nothing to do with App Store screening.
2. The monopoly in this case is not whether iPhones have a monopoly against other phones but rather if Apple itself has a monopoly in providing for apps on its device which it does. It is not illegal to have a monopoly despite common understanding of such in the general public. It is illegal to use such a position to the detriment of the consumer. So, if Amazon with a monopoly on book distribution squeezed the publishers to lower prices on books, it is not illegal. This is why Apple lost the case claiming its collusion with publishers was to increase competition and lower prices against a monopoly. It was colluding to have the effect of increasing prices to the buyer by squeezing Amazon out from publishers.
Not the App Store monopoly (which is legal) but the use of its position to result in higher prices on the iPhone (which is subject to anti-trust) is what was being claimed in the original case with buyers of iPhone as plaintiffs. Apple would find it hard to defend against that, so they are trying to say the consumers cannot bring the case but developers can. This is what was decided by the Supreme Court. There is no monopoly for developers since they are free to develop to any ecosystem or not and there is no separate condition of sale for them to require/demand alternates to App Store. That is what Apple is counting on.
iPhone buyers have a very strong case, since having bought the phone they are locked into that monopolistic practice. Just having bought the phone does not allow the manufacturer to submit the buyer to monopolistic practices within it. It is neither an implied nor explicit condition of sale and would not be allowed as such.
But then we are in such a precarious moment of history where separation of powers is only superficial and so a meeting of Tim Cook with the President and a quid pro quo agreement for Apple to make a public announcement of bringing jobs back to the US that plays well politically can very well change what the SC majority may decide on the actual case. So, not even the legal experts can predict what the outcome will be.