Better brush up on you see and avoid, it is the only game in town for the time being
I think that is a little widesweeping, although I understand "where you are coming from".
Although I use TAS most of the time (which is hellishly expensive) I also have used PCAS for many hundreds of hours. In reality both "see" a lot of aircraft that you would not see with mark 1 eyes.
In fact whilst I note your view on the ratio of transponding to non transponding traffic I am constantly surprised how little traffic is reported when receiving a RIS that is not apparent on TAS. Perhaps, as you suggest, the ratio changes dramatically at the weekend and is dependant where you fly.
Never the less combine your eyes with PCAS or TAS and you have a better chance of avoiding other aircraft. Moreover, the nearest collision I had was between me (in a twin) and another twin. At these speeds the time to see and avoid becomes unpleasantly short, BUT the faster the traffic the more likely it WILL be transponding. The threat (to some degree) is proportional to speed.
In short, do not dismiss safety aids of this sort so readily. they may not be a panacea, but I believe they are a significant aid.