You are almost making my point for me.
I see GDL as a distraction and as we do already in effect have that, legislators would be better spending their time researching other measures - which I would speculate as being more effective.
GDL is tinkering around the edges, whilst maintaining the status quo and placating the motoring lobby.
What we need, is the road safety review that was promised by Government an age ago.
I'm not really arguing with your viewpoint, and accept that GDL is not a panacea but I look upon it as a measure that can be enacted quickly (maybe within 6 months if Government get onboard with the idea), whereas a road safety review or restructuring of training/testing methods and standards will take years - maybe even decades - to be undertaken, analysed, discussed ad nauseum in Parliament and finally enacted in some watered down version. In the meantime GDL would save lives and there can be regulations tied to it around driving modified cars, engine power, carrying passengers, driving times, etc.. The risk of new drivers breaking those rules will be the same as that for breaking existing regulations.
The upside of introducing GDL may not be great but it's better than the downside of not doing it, IMHO.
BTW, I agree that we need a review of road safety and training/testing but I don't see this (or any other Government) taking it on and seeing it through to actual, meaningful changes.