There's no reason what so ever that you cannot build touch friendly apps on a fuller OS.

It's hard to adapt an existing OS to a world of small touch screens. But if you do it that way, you get a so much more advanced environment.

There are advantages to simple environments, and to advanced ones. Potentially, a simple environment will always be faster and an advanced one will always be more powerful.

Either way, you would want the GUI to be optimized for mobile use. But you'd also want to be able to use regular software despite nobody caring to optimize it. Especially since smart phones of this age can be used as a laptop, complete with screen, mouse and keyboard.