A lot of people forget, but iOS shares the same underpinnings as MacOS X. In fact, when Steve Jobs announced the original iPhone, he didn't say it ran iPhone OS, he said it ran MacOS.
I wouldn't be surprised if there's macOS already ready to go for ARM, much like there was an Intel MacOS for years before the transition was announced.
As for whether iOS could replace macOS, if you go back to my first point, think about iOS as a version of macOS that was stripped down and has been built back up over the intervening decade. I don't think they could make the switch tomorrow, but in a couple of years, it's not unreasonable to think there could be a reunification of the iOS and macOS codebases, creating, essentially a fully-featured iOS for desktop hardware with the features we've come to expect from modern desktop OSes.
I wouldn't be surprised if there's macOS already ready to go for ARM, much like there was an Intel MacOS for years before the transition was announced.
As for whether iOS could replace macOS, if you go back to my first point, think about iOS as a version of macOS that was stripped down and has been built back up over the intervening decade. I don't think they could make the switch tomorrow, but in a couple of years, it's not unreasonable to think there could be a reunification of the iOS and macOS codebases, creating, essentially a fully-featured iOS for desktop hardware with the features we've come to expect from modern desktop OSes.