Gpn, I don’t think you’re quite right about everybody needing it. AIUI, Zaon etc. requires everyone to have a transponder on, but not necessarily to have their own Zaon. Flarm, on the other hand, does indeed need everyone else to have FLARM to be detected. The full scale TCAS requires everything else to have a transponder to be detected, but only gives automatic conflict resolution to mode S equipped aircraft (when the two TCAS’s “talk” to each other).
No doubt an expert will be along to correct me if any part of this is wrong.
Chris N.