I think what killed on-road racing is it is kind of the "F1" of r/c racing in where you have to spend the money to be competitive. It's the same with any form of racing of course but more so with on road, especially classes like 1/10th, touring, 1/8th nitro on road etc...
On the other hand, there are budget friendly on road classes, but they are more for fun rather than to be seriously competitive.
I guess you could kind of think of it as anything really, it comes and goes in waves. 5 years from now 1/10th could be bigger than 1/8th buggies, but I don't know how likely that is if the costs don't come down.