Rod1 - I have to say I have never understood this obsession with mode S other than regulatory conformity. As you say mode C transponders can be purchased for very little. That largely eliminates the cost argument.
The weight power aspect is more difficult. I dont think there is really any significant weight saving between a mode C and a mode S unit with both offering light weight alternatives.
I wonder what percentage of the fleet could / would fit a mode C transponder if they were required to?