Apple is leveraging their success in the operating system space to control competition in other markets and to limit options available to consumers. This is classic anti competitive behaviour and has nothing to do with whether the OS is sold separately or is bundled with the phone. There are very few hardware advantages of iphones vs. equivalently priced (say) pixel phones, especially not ones that don't rely heavily on software support (eg cameras). So in practice we already are in the reality you imagine anyways.
