Simple, and not true.

Each task has it's own best form of input. Period.

A device which wants to do well at that task, will have an input mechanism that is well suited to that task. Period.

For some things, a physical keypad is better than virtual buttons (and even without that, a device which doesn't reduce the usable screen space by making you use a virtual keyboard will always be less useful than a device which lets you retain the full screen while also giving input; can't do that on the existing iPhone ssh clients, for example, so you're left with a cramped and annoying ssh experience). For some things, a fine tip stylus is better than a finger. A device which doesn't give you those options will be suboptimal for those tasks which excel with those input mechanisms.