I imagine the used market would be flooded with "iPhones" filled with non-genuine parts. Non genuine parts often don't interact well with things like firmware updates.

So by this logic all automobiles should only be serviced by the manufacturer. Somehow I think we'd manage to figure something out for a product that is orders of magnitude less dangerous than an automobile.

That's already the case for the more complicated parts. Also, at least here in the EU, if you want factory warranty you have to follow the dealer schedule and do the necessary replacements.