only to the big airplanes (whose lives are presumably more important). How would two non-TCAS equipped aircraft be made aware of a conflict risk ?
Define big....i don't fly big aircraft, but i do fly commercially and do see the need for the equipment. Nobody's life is more important than any other, hence the need to make it as safe as possible for all.
As for seeing conflict risk, one of the types I operate does not have TCAS, therefore we like to use a radar service particularly above FL100. Below that, i regularly transit VFR, see and be seen applies, it's a choice and i don't expect everything to be squwaking or recieving a service. But in controlled airspace and above FL100 (where the speed restriction is lifted) it should be mandatory to carry a Mode S transponder.