I do not think many onroad racers make a switch to offroad. If you are an onroad fan and can not pay the high costs then you take a search for another onroad class.
Offroad is booming because most young drivers choose an offroad car as 1st RC car because they know the can drop it everywhere and take a drive. They grow up with this and later on a part of those starters will end up in the offroad racing scene. Beside that, it is less difficult to find a track close to home.
Onroad racing doesn't have to be expensive, I think Edam with their RTR 1/8 car does show that, it is a good choice for most 1/8 onroad starters to become an expert on clublevel racing. The high costs are more within the mind of the driver. He wants to have the new 900 dollar Murnan tuned Nova Flash PTS and all fancy titanium, carbon and aluminium upgrades and also lightweight rimms and bodies while most racers do not need it, most drivers do not gain an advantage with all those expensive and short live stuff. If you are more than a second off from the fastest laptimes then the fingers and the setup must do the trick and not the option parts.
I also do think on these forums there is less common sense. If a starter asks what engine he needs he will be pointed to many expensive engines and mostly modified while a simple cheap 5p or 7p engine will do fine but no, most people are telling him to get the Murnan Flash or something in that order.
The high costs are also really there but are also partly made by the users on these forums advising the wrong stuff making the starters thinking in the same way.....