So I guess you didn't read up on Xamarin.IOS then... The simulator is available remotly and could be hosted by Apple as a service. Same for the sdk and final build environment. Apple doesn't have to make anything running on windows.
So late reply but...
Running a debugger over a local network is tolerable.
Running a debugger over the internet for an iOS app would be insane.
Cloud apps have remote debuggers, but iOS apps are way more data intensive. A typical build/debug cycle is usually about a few seconds, a minute or two at most. Can you imagine waiting for a 1 or 2 gig app to upload to the cloud before you can debug it?
There are also Xcode features that depend on the simulator being local. And stuff like Playgrounds go away too. (Although there are less powerful cloud solutions for that.)
And just the amount of information that round trips during serious debugging sessions is significant.
If you're Apple, and you sit down and look at the sort of cloud rollout and software development you'd have to do, it's just way easier to update the Mac Pro, or decide you don't care about high end development any more.