A traditional barometric altimeter is:
- Relatively cheap.
- Virtually indestructible.
- Needs no power source. Draws no load from the aircraft electrical system, battery or engines.
- Will work after a complete electrical failue.
- Sufficiently accurate to provide safe separation.
- Easily adjustable between required datums: QFE, QNH, SPS etc.
- Pressure altitude not true altitude is directly relevant to many aircraft performance calculations.
- Many users of the atmosphere do so without any electrical systems (eg some vintage aircraft, microlights, balloons, airships, parachutists, skydivers, mountaineers! etc etc.
- Transponder altitude reporting is based on pressure altitude on the 1013 datum not true altitude.
- Robust. GPS signals can be easily jammed or spoofed.
From experience in aviation some of the guiding principles are: if it ain't broke don't fix it, keep it simple and avoid change for the sake of change.
In my personal experience GPS altitude is often not particularly accurate and is sometimes not available at all.
Now given what I've written, perhaps you could offer some good reasons why the industry should choose to use GPS altimeters instead of barometric altimeters. Go on... give me ten good reaons.
(I can think of a few, but a baralt still does exactly what it says on the tin and I think is overall the best option.)