There are already many projects (successfully) trying to get better operating systems running on iDevices. Project Sandcastle got press coverage a while ago for porting Android to the iPhone. That work was also picked up by postmarketOS contributors who have gotten Linux limping along on some iPhones. Those are just the ones I’ve heard of, I’m sure there are others.

You are right though. Apart from the CPU, there’s basically no documentation for any other components in these bricks, which means that these projects are limited to booting and maybe displaying something on the screen, and only for a few lucky devices. I wish governments would require companies like Apple not only to easily allow sideloading alternative operating systems, but also to foster their development, but that’s just a fantasy and will likely never happen.