You listed windowing/UI concerns. And, while they may commonly come bundled with the OS, they're not the same thing.

They're a platform concern.

Not only does the OS UI reflect the input method, every single application you run does as well. A touch-optimized application will be clumsy and primitive when using a mouse; a mouse-optimized application will be miserable when using a finger.

If an application can be written once for both, often it'll be a poor compromise or only properly support one of the two input methods.

Look at how awful early Java "write once run everywhere" applications were. Yes, some of that was the Java platform itself, but developers were given the opportunity to ignore platform-specific UX concerns and many eagerly embraced it.

Again, the problem is pretending to use the same UI for all input methods. And I agree that is a mistake.

But what if you could use the same language and frameworks on all devices? The same tools (IDE, code editor, compiler, etc)? That's what I'm referring to.

I think a single platform for all our devices: mobile, tablet, desktop, etc, is a mistake. The word platform means more than just the OS and includes the entire L&F/experience with the UI.

I think a single OS for them all is fine.

Consider gnome+linux vs android+linux. They're both linux, and they're not the same platform.

By that definition, I could argue Apple got there years ago by using the same core kernel and frameworks for OS X and iOS.

That's clearly not what the original poster was arguing.

Well, it's the definition. I also believe that OS X and iOS have diverged somewhat in the kernal/framework space.

iOS and macOS are much more different than a change of UI.

The UI can adapt to the input devices, that not an OS concern.

The UI is but a small part of an OS or an application.

It's obvious that the UI has to be different depending on the device, input, etc.