Maybe it should be a choice to have TCAS but compulsory to have at least mode C transponder for all flying machines?
Remember, we as a community, or at least certain parts of it, have "successfully" fought off (at least delayed) "compulsory" transponders for GA. You might say the fight was over Mode S v Mode C, but in practical terms the effect is the same... (quick Google shows an A/C Xpdr at £1, S at £1.5K - each + same installation costs).
The problem you "fight" is that you, with a TCAS / PCAS / Skywatch / FLARM or whatever are expecting / relying on someone else to purchase / fit / get approved a device to make yours work. Some of the latter would, maybe quite rightly, ask you to pay for it?
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I do not know the numbers who have Mode C / Mode S / FLARM / whatever, but for collision avoidance purposes, especially given the lack of radar service(s), we are surely a long way from the "critical mass" to significantly reduce the chance of a mid-air?
NoD
PS before it gets personal, we are just fitting Mode S, but for other reasons...