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?
That depends on where you fly. If you fly at 1000ft then I agree 100%. If you fly at 3000ft+ then I am sure nearly all are Mode C/S equipped and transponding.
From my 900hrs of flying, with as much under an RIS as I can get, of the "level unknown" i.e. nontransponding contacts that I recall spotting, countless contacts were below me, and
none that I ever recall was above me. (Of course the majority I never spotted). The statistical picture here is overwhelming and that is why I sometimes appear a bit cynical on this forum about the correlation between who is flying without a transponder (often a homebuilt/microlight type) and how low they fly, and probably how far (I mean not far) they fly.
Practically everybody who flies seriously, doing real distances and using the available airspace to the full, has a Mode C at least and uses it. But there is a large and vocal group who are against transponders but who probably do little beyond local bimbles, at low levels.
(time to duck under the barrage of examples of adventurers who flew their motorised hang glider all the way to Kathmandu, etc)
Backpacker - I don't think airfields generally make good waypoints. I would challenge you to to a dead reckoning exercise with say 10 waypoints, all of them being grass-only ones. Some are damn hard to spot for a visitor (easy for locals, obviously). And the bigger ones have an ATZ so you have to be at least 2000ft AGL and preferably more due to overhead joins at 2000ft, and you ought to talk to them. I never use airfields as waypoints, in the GPS route. They have minimal visual benefit - unless it's Heathrow
