Why would it decimate the Windows market? From my experience, there's a strong correlation between iPhone and Mac usage.
Looking at the stats, the Win:Mac ratio is 4:1 but Android:iPhone only 2:1 so it might hurt Windows. But if iPhone users are more likely to use Mac or don't use computers much already, then expanding iPhone capabilities would cannibalize Apple business.
No, the iPhone has over 50% market share in the US, macOS is nowhere near that.
Because then most people with an iPhone wouldn't need to buy a separate laptop/desktop. I'm sure Android as well would follow in short order (not the half hearted attempts they've made so far). Sales would plummet. Windows decimated.