Excellent list Whirly.
I would also add UK weather - I know many here will disagree but it's pretty hard to fly fixed wing without instrument privileges. If I couldn't I would pack it in right away; there are just too many clouds about
Even if most flights can be done in VMC on the day, the legal need to be VMC is very restrictive.
Many people say "one can fly everywhere VFR" but most of them are people with loads of time on their hands and living close to an airfield.
A recent press report said 65% of NPPL applicants are existing PPLs who can no longer pass their CAA Class 2 medical. If accurate, I am sure this was not the intention.
If one reads between the lines of the list, doesn't most of it come down to most people entering GA not having enough money to pursue this hobby to start with? That's the REAL problem.
There are loads of people with money but most of them don't go anywhere near GA. The anorak nature of the social scene is enough to put them off.
The whole scene is in dire need of modernising, and then somebody could set up a well managed well funded well equipped (ie no Cessnas or Pipers) flying club, attracting people who can actually afford to get their training and still have some beans left to fly somewhere further than they can drive.
There is a need to recognise that there should be two groups of flyers: the vintage types and the modern types. The vintage scene is amply catered for by articles in GASIL showing how to place the tail wheel in a plastic bucket to stop mice climbing up it and eating the seat covers
The modern scene isn't catered for much because its membership is too small. Of these two the vintage scene can hardly be expanded; the future lies in focussing on and exploiting the other one.
Sadly, even those few organisations that have got modern planes have failed to point their marketing (if one can call it that) at the more affluent bits of their local population.
There is a lot more one could do...
One thing is that a lot of schools don't like PPLs to hang around, perhaps taking students up and "giving them bad habits". This and other factors means that a new PPL is very much on his own. I was an aircraft owner by then which kept the incentive going but most people just continue to rent. The best thing is to quickly get into a syndicate; the problem is finding one with a half decent aircraft, which takes us right back to there not being enough people with beans.