I have clearly been a little bit lazy in dealing with temperature in my second post.
Changes in temperature at a constant pressure level will not change the relationship between CAS and Mach Number.
But changes in altitude with change the realtionship between CAS and TAS (caused by changes in air density due mainly to changes in static pressure). So if we climb or descend at a constant CAS, the Mach Number will change.
The effects are illustrated in the attached diagrams.
As the bottom diagram shows, VMO is the most limiting speed at low altitude and MMO is the most limiting speed at high altitude.
As stated by JT, at high altitude it is far easier to monitor Mach number than it is to continue to monitor CAS and need to apply a constantly changing value for the speed limit.
