MacOS can now run a lot of IOS applications. They have icloud to sync a bunch of things. But only for their own applications. But maybe it's more clear if we limit the discussion to just phones. The key question, why do people only buy 1 iphone? Is it because it's perfect for every situation? Or because they can't play nice together?

Switching from one to the other is a PITA. What if that was seamless. You just pick one up and now everything is there. You might have a phone for work, going out, hiking, etc. You might have phones matching different iphones. Apple could be selling you many phones. Losing a phone would be a lot less dramatic because you'd have multiple spares. Do they all have to have the same screen size? Do all of them even need a screen?

Apple sells lots of phones. They could be selling 2-3x more. That's just phones. The way they positioned their VR device and watch is kind of telling. They are both kind of satellite devices. But apparently that requires special apps and a new OS (for both).