forgive me if I have missed it
Back to the original post, do the instruments which indicate mach number do so by direct or indirect reference to the standard atmosphere (ie is it calibrated in from a model of the world as opposed to being directly measured?). Didn't quite get an understanding of this before the discussion zoomed off in search of the missing 'T'.
In terms of 'why mach number', isn't it just that at low alt the margin between TAS and LSS is large enough you can ignore compressibility but as altitude increases and TAS converges on the decreasing LSS you have to do things more rigourously (ie take into account local variation in flow speeds, transient shock formation etc ) ?