predator - I agree with your points about people keeping their imports.
The shortest I kept mine for wouldve been about 3 months. The longest about a year.
You are wrong about not seeing R32's etc around. Keep in mind there are a shitload that are still for sale, many are still at the docks here and in japan. You not only have 89 model skylines, but also 180sx, silvias, and other stuff like older pajero's. vans, porsches, etc. And other stuff that has come in from the states and europe etc, would also be classified under the 15 year rule.
So ultimately, yes, there are a shitload of older cars on our roads now. 15-20000 cars isnt really that much considering there probably would be around 10 million registered cars on our roads ?