boomerangben
I very much doubt that anyone spends more than 10% of their time "playing with the GPS" (or whatever people who navigate using "non approved" methods get accused of).
I don't think there is an answer which can be presently and immediately implemented. What could have been done, say 10 years ago, was to have made Mode C mandatory. Transponder technology is mostly 1970s/1980s and low power transponders suitable for gliders and such would have been developed way back then. There would have been huge fuss of course, appropriate to any occassion when a GA pilot is asked to spend more than £10
but the fuss would have died out by now.
And then, somebody who wants reliable traffic warnings could buy a traffic warning system. The prices of those would in turn have come down a bit, perhaps down to £5k.
The remaining pilots would have to look out of the window, or just keep their fingers crossed; either is probably equally effective.