I really hope onroad makes a comeback too! I love driving onroad much more than offroad, but I do enjoy both. It seems that there are few drivers that do both types in my area. There are strong onroad and strong offroad guys. It seems that the best way to grow onroad as a whole is to get strictly offroad guys to try and run both. How to do that? Not sure, maybe better marketing/advertising at the track.
Yes, onroad and offroad both have many classes. But the cool thing about onroad is that many of the TC classes all have the same base chassis and require only a different motor/body to run in a different class. Obviously you can't switch them throughout the course of the day to race in multiple classes the same day, but from week to week its certainly possible. This is very far from the case in offroad where you need an entirely new chassis to switch classes. Which is really striking because you also need large amounts of tires/wings etc to run offroad, making it more expensive.
As far as the multiple classes on track at the same time idea, that was actually tried in my area recently with relatively good success. There are quite a few threads littered around rctech. Here's the facebook page:
https://www.facebook.com/unitedsportscarrc