It must also be remembered that the CAS airspace grab proposals by CoventryThomasCook and Robindncasterfinningleyhood would deny the use of airspace for those who currently use it for more than just 'transiting' under a RIS or RAS......
The main advantages of Mode S over Mode C are reduced susceptibility to 'FRUIT' and 'co-ordinated' RAs for TCAS equipped aircraft. But TCAS will give RAs against Mode C, of course - but only TAs against Mode A which are a definite hazard as they could be masking a real RA requirement or generating a TA when neither TA nor RA are genuine....
The 'selective' nature and unique identification features confer no benefits to TCAS-equipped aircraft. Nor do they constitute any valid reason for mandatory replacement of serviceable Mode C equipment.
So yes, when the regulations change, let's make the transponder proposals
appropriate to the aircraft and airspace categories, require all transponders to include altitude information and require any new trasnponder installations to be Mode S - leading to a gradual phasing in as older Mode C units reach their end of life point.
And Lo-Co Chavair drivers should remember that IFR separation from VFR traffic is not required to be provided in Class D airspace in VMC. All you are entitled to is traffic information on request.
Also, 250KIAS below FL100 is mandatory outside Class A airspace
!!