[zelly]

A mobile app written in portable code that runs without emulation on a desktop.

The desktop OS would expose the same platform SDK as the mobile OS. (Needless to say, apps depending on the accelerometer, selfie cam, etc. for major functionality would not work.)

This is already the case with the latest generation of Macs. No speculation necessary. It's possible and has been done. Also Chromebooks can run Android apps.

What remains to be seen is when (not if) Windows would mimic what Apple did, as they have been doing for the past decades. I suppose the easiest way would be to run Android (and Google Play) on WSL. Android and Google Play can run on x86–no need for emulation. How clunky this turns out is how willing MS and Google are to work together.

In the end there will just be two platforms you would have to support as a GUI applications developer: iOS and Android.