AFAIK, AF447 had altitude read-outs at all times- it was airspeed that was in error.
I do agree that a read out of GPS altitude might be useful in some circumstances- more so that Rad Alt, as that is actually a reading of height above terrain, not altitude.
Indeed, the smart landing "Altimeter Setting" system works by comparing Baro to GPS altitude.
Similarly, the AF flight WOULD have had a ground-speed reading from it's inertial system- which includes GPS. but ground-speed, as has been pointed out, is of little use as a flight instrument at altitude, and without an air data source, there is no way to calculate IAS or TAS.