I disagree. Layering a new UI on top of an existing OS is a lot of work, but ultimately thats all it is - another layer. What iOS and Android do that is fundamentally different to OSX and other desktop OSs is not so much to do with the UI, its to do with quickly killing or hibernating any process that isn't being used right at that time.

When you learn to write an app for iOS or Android, the first thing you learn is that your app needs to be ready to freeze and save state at a moments notice, and then re-hydrate itself later, also at a moments notice. This is the fundamental difference in the OS, not the touchscreen and widgets.