Back in the PC days I posed the question: What if Apple sold its computers with options for the OS, e.g., [ ] Apple-provided [ ] User-provided. In other words, what if one could buy just the Apple hardware, without a pre-installed OS. I used to run NetBSD on the Mac. I did not care about getting 100% support for every feature of the hardware, I just wanted a working computer. It is funny because today iOS is the mandatory Apple-installed OS and it borrows heavily from BSD. The old AirPort Extreme even ran a NetBSD 6.1 kernel. If there is line somewhere between "convenience", e.g., supply of a pre-installed OS, and "loss of meaningful ownership", e.g., ceding control to a seller even after purchase, it seems to me that Apple is way over that line on the side of denying meaningful ownership. If iOS is truly valuable, and I have no reason to believe otherwise, then sell it separately and let purchasers decide if and when to purchase, install or uninstall it. If the buyer does not want to use iOS then why should they have to pay for it. This question, as boring as it is, still triggers HN commenters, so there is obviously something coveted about the obligatory bundling of consumer hardware with proprietary OS, in spite of all the non-proprietary options.
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.
They do (and Windows on the older x86 based devices), but since the move to Apple Silicon, combined with a lack of availability of the specs, it becomes harder and harder. But OP is talking primarily about iOS, where it is impossible to run Linux (or any other OS)
It becomes harder every time apple makes a change to something. GPU, bootloader, some secondary processor in charge or some obscure but surprisingly important thing. The whole thing is dependent on Apple deciding not to disallow it for now.
