Unfortunately GPS/INS derived information does not provide all the information supplied by the air sensors on the aircraft to allow the aircraft to be flown safely, as aircraft fly relative to the airmass around them.
These sensors (pitot/static, Angle of Attack (AoA), Total Air Temperature (TAT)) provide information on what the aircraft is experiencing in the airmass it finds itself in. They are the only means to provide information that can be used by the automation or crew to determine if the aircraft is flying within its flight envelope (such as Airspeed, AoA, max pressure altitude). No GPS/INS sensor can do this.
Furthermore, aircraft instruments are calibrated for the ICAO standard atmosphere - to ensure that all aircraft fly to the same standard and so there are no conflicts due to altitude. This 'standard atmosphere' is based on pressure altitude and other compensations (such as temperature). It would take a significant global effort to change the concept of the ICAO standard atmosphere to mean GPS altitude, including changing the equipment fitted to every airworthy aircraft in the world.