While I would fully acknowledge that
"perfectly content with bouncing around in something like a tigermoth or cub which has minimal instrumentation"
is true and there is nothing whatever wrong with it, I'd suggest that only a tiny % of the participants in the aforementioned side of GA will hang in there for very long, which takes us back to the context of the thread: why do people drop out.
Of all the people who would (given the money, time, etc) like to learn to fly, the long term satisfaction of bimbling about is pretty minimal. A few flights, with one's mates, then a few more flights (without one's mates), and for most the novel way of spending £100/hour gets a bit tedious.
All the time one accepts that bimbling about on nice days, without going anywhere too far, is just fine, one will be looking at a massive attrition rate.
Which doesn't matter, provided that new punters continue to pile into the training machine at a rate of thousands a year, each one dropping £8000 or so at the local airfield as they pass through.
Understandably, that's how the training industry has always judged success - how many punters spend how much money. What they do afterwards doesn't matter.
What I am saying is that to lift GA out of the rut, one needs to reduce the attrition rate, and that can't be done unless one does something beyond just running the sausage factory.
Even a very small reduction in the attrition rate, from say 90% to say 70%, would dramatically increase the number of participants in GA. It would result in an even bigger increase in the number of people who can spend some real money because they are the ones who have deserted it in the very recent decades.
More courses are all very well but without extra privileges almost nobody will turn up. The converse also applies: look at the absolutely massive amount of work, time (years), money, hassle, and more money some people put in just to get a FAA PPL/IR and what is it all for? Basically, just the ability to fly on an IFR clearance outside the UK.