Just one of many, really
Voice control as one of the many user interfaces. What will liberate (not kill) the app market is the complete freedom to interface with any app with voice control being just one voice in the crowd. Maybe I want specific gestures, automation, external sensors, mind control, a mouse, normal touch, secure locks, childproofing etc. Same reasoning on output, flexibility in how and where to display things will have to grow as well.
Standards will set us free. Single interface designs are pipe dreams. The world of form is legion and will always demand more ways to access it, not less.