Surely the point of the Mode S comment was that everybody will then have to have a transponder with Modes A and C as well, so there should be no non-transponding traffic for it to miss?
I would check this out. I think (should damn well know, having a GTX330) that you can switch off altitude reporting on a Mode S transponder. This is bound to be done by some d1ckheads who want to be able to bust airspace vertically but who have to carry Mode S.
A transponder not returning altitude is useless for traffic avoidance purposes.
Don’t assume Mode S will be universal, the French have just exempted 8000 microlights and most of Europe will do the same.
Another reason for not spending money on TCAS then
VFR aircraft collisions are pretty rare
Rare but not zero, especially around busy GA airfields of the "free for all" type.
Practically nonexistent in IMC, presumably because so few GA pilots fly in IMC.
b) it may be inoperative, and transponding is not yet mandatory
c) there may be an electrical load issue
b) is a bit selfish IMHO. The pilot may as well paint a big finger sticking upwards on the side of his plane. c) is pretty unlikely if the thing has actually been fitted!!
So, the statistical case for TCAS is pretty poor. It's better to have a warning (assuming the direction is provided) than not, of course. I would just spend the money on a very very good GPS, not the usual piece of junk from a camping shop - that will do more for one's flying.
IF Mode C was mandatory then TCAS would be great. I think this is unlikely to happen in the UK (Mode S for all VFR traffic) because of the technical problems with equipment carriage and power.
I don't believe in a lookout being very useful. I know a lookout is one of the cornerstones of aviation (alongside dead reckoning and leather helmets and goggles) but assuming straight line trajectories a target on a true collision course will be a stationary point until it hits, so those you spotted were in fact not going to hit (no matter how close they seemed at the time). I look out very carefully for gliders, and try to fly above clouds where they won't be anyway.