The big power of GUI is that it was an alternate control scheme to do anything you can do on the CLI. Voice control is not a UI. It's a limited set of functions not meant to offer analogous control over any aspect of your device really; if it does it's happenstance. That's why Siri is for setting alarms and not doing complicated workflows that would require your thumb. I wish it was a voice ui, though. I wish I could stick my phone on a shelf and step back and tell Siri to take a picture, to do that function on the phone my thumb can do, but the best Siri does is open the camera app because the voice controls are just that, a limited set of control levers to pull and not a complete user interface.