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.
Unless I misunderstand you, that would very much suprise me, because when flying under a "traffic service" the majority of reported contacts are "level unknown" which means either no TXP or Mode A only.
These contacts would not usefully show on any TCAS system.
I think Mode C fit % is close to 100% among those flying higher up, say 3000ft+. It's a whole different pilot population up there.
Between flying below 2000ft, and flying above 3000ft (approx) it seems as if GA exists in two different compartments.
A lot of pilots joke about getting a nosebleed at a few thousand feet but I don't think many of them are actually joking.