The decline in on-road classes is because Tamiya no longer promote their scale 'moving models' in the way they used to. Try most model shops and you no longer see the huge rows of Touring Cars with the twin stars on the box.
New blood arrives in on-road through people being able to build and drive cars out of the box. None of the pan cars on sale today can be raced out of the box. 12th grew at its fastest rate when Tamiya made the Porsche 956 on a pan car chassis, and declined at its fastest rate when first the Sand Scorcher/Rough Rider came out, and later the Nissan Calsonic Touring Car.
People could go into a model shop, buy a car, build it and race it. Today, they can't in on-road or oval, but they still can in off-road thanks to the many RTRs from AE, Losi and Traxxas. It is no wonder places rip up the rug and pile in the dirt - that's where the new racers are coming from.
If people who see a bunch of Rules for a new class (WGT) then decide they don't want to stick to them and race something else, why is anyone surprised that the class dies??!!
Pan cars of any description are for the racer who knows stuff, not for the newcomer. Having no TCs in Tamiya boxes will never bring the new guys into TC. Until that changes, off-road will always get the new drivers, and we can only hope they come and try the on-road classes and stay with us. Sad, but true...
