As the reviewer in the Pistonheads article says VW has started to cannibalise itself. By bringing out the Golf R, Golf GTD, Scirocco 2.0 TSI and the Scirocco R these models have all chipped away at GTI sales. Add into the mix other hot hatchbacks like the Ford Focus ST, Vauxhall Astra VXR, Renault Megane Renaultsport, Mini Cooper Works and the BMW M135i it's no wonder sales figures are down for the Golf GTI.
It easy to forget but the likes of the A3, A-Class, 1 Series and Seat Leon weren't even a pipe dream when the Mk1, Mk2 and even Mk3 GTI's were in their pomp. Often copied never equalled eh.....
Bottoms line is there is far more competition out there now, lots more choice to temp people to spend their hard earned cash elsewhere. Also once you've thrown in a few options on a GTI its creeping towards £30k. Which if you pinch your self is a lot of money. This doesn't mean the GTI product is inferior, quite the opposite, it sounds like VW have tried really hard with the Mk7 and its getting rave reviews. Good luck to them, but, the market is looking very crowded.