Rivet gun,
CAS was invented as a calibration standard for the mechanical airspeed indicator. As the mechanical ASI is consigned to history, so should be CAS.
CAS is determined by taking the compressibility of air into consideration. Until the physical properties of air change there's absolutely no reason to discard the present calibration law. EAS on the other hand is not a physically meaningful quantity. EAS tells you absolutely nothing about compressibility. It is a fictitious speed resulting from the setting of rho to its ISA MSL value in the dynamic pressure equation. The equation for dynamic pressure is a solution to Euler's equation (which is a reduced form of the Navier-Stokes equation which itself is a reduced form of the Burnett equation, etc) where density along a streamline is regarded as staying constant.
CAS on the other hand is developed from a solution (the Barré de Saint-Venant equation for the subsonic regime) which treats density along a streamline as varying according to an isentropic law. CAS does not tell you anything about dynamic pressure.