TCAS would be great but until Mode C is made mandatory it will be of limited usefulness for avoiding actually possible collisions, versus the cost of the proper active system - £10k+. And I suspect a huge % of the low level bimblers don't have a transponder or don't switch it on, anyway, and if you fly high, there ain't nobody there, more or less.
IO540
I think that is the one aspect of your post with which I disagree, although I havent got any proven stats with which to back it up.
Using TAS I reckon that only around 10% of primary contacts reported by
PR is not shown on TAS, and about 5% are shown with no height information. That would suggest that most traffic has transponders at a mix of levels - albeit I agree the number without is even less at higher levels.
It would be interesting to know what % of the UK GA fleet has a transponder fitted - does anyone know?