Okay, I mentioned desktop because you mentioned "personal computer industry" which I assumed means desktop/laptops.
I think there are indeed more private (some can even be more secure) alternative to iOS, like calyx and/or graphene.
But like you said, they do require a reasonable amount of computing literacy to install: first they need to know these projects exists, then they will need to connect their phone to their computer and click a single button.
Thus, I think there is indeed no private and "popular" alternative to iOS, that a completely tech illiterate person can easily obtain.