Someone also pointed out (but you may have missed it) is that above a certain altitude, the only thing that's important is relative altitude. Which means: not the altitude above terrain, but the altitude that's between me and that other airplane (or other user of the air).
Pressure drops as altitude goes up. I don't think anybody disputes that. So the best way to ensure separation from other aircraft is to assign each aircraft a different air pressure level on which to operate.
Now for convenience we add a few gears and things, and represent that pressure level as an altitude but that's just to prevent us from having two separate dials in the cockpit.
Unless you found another simple, reliable and foolproof method of ensuring separation between two aircraft, barometric altimetry is here to stay.
Is using pressure related instruments still the most accurate and best way of reading your altitude/height in aircraft?
If height above terrain is a critically important factor, eg. on a ILS Cat-III autoland approach, you'll find that the barometric altimeter is NOT accurate enough and has already been replaced with, for instance, a radar altimeter.