Why is the Inertial Reference System not used to determine true altitude? Given three very accurate laser gyros and accelerometers I believe the Inertial Reference System (IRS) should be as accurate in determining true altitude as it is in determining lat/long, providing the aerodrome elevation was supplied at initialization.

I realize that pressure and density altitude are what is used in aviation, but could the IRS be used as a backup? Could it replace or backup a radar altimeter for approach/landing?
IRS will drift over time. Decision altitude is 200ft for ILS CAT I approach.  0.5nm drift is not to much for navigation but it will be huge difference in altitude and you can hit obstacles if your altitude has drifted just couple of feet. Probably it could be used if backed by GPS as EGPWS does or non baro RNAV approaches do. But even GPS precision alone is not enough in this case and you need WAAS.
Radio altimeter can give much better precision.