I'm pretty skeptical of the notion that iOS could replace macOS any time soon. Here's an experiment if you don't believe me: grab a mac, run the iOS emulator on it full-screen and try to do all your normal tasks in the emulator, without jumping out to the desktop. Let me know how that feels after a few hours.

You could argue that with some engineering time they can add some affordances to the iOS interface to make it more pleasant on desktop. Sure. But every app is also going to need to adapt for this -- they need to drag along the entire ecosystem. Honestly it would be way easier to make macOS run on ARM than it would be to make iOS run on desktop. They already did this whole processor shift thing once and it worked out just fine. Just package things in universal binaries and emulate the old CPU for legacy software. Not to mention with something like LLVM it's not crazy to imagine distributing applications in a near-native bytecode that gets JIT'd to the target machine.

I know microsoft has gotten pretty close to accomplishing the entire one-os-for-everything model, but look how much time and money it's taken them, and the amount of good will they burned through getting there.