[bandwevil]

You'd need to be able to ensure that everyone is using GPS altitudes before requiring it, and there are some pretty ancient planes still flying out there. As far as I know, ATC radar doesn't actually measure altitude but rather relies on the plane's transponder to report it.If you have some mix of baro and GPS altitudes, you can no longer safely ensure separation.

There should be enough checks for setting the proper baro setting - both pilots check the setting individually when changing it and it's reported to them a number of times throughout descent. Even if they do manage to get it wrong through all that, the GPWS should notify them if they get too close to any obstacles.

[Alupis]

And not everyone uses GPS (GLONASS is a competing system used by some parts of the world, specifically developed by the USSR to not have to rely on a USA based system).