Fundamentally there are difficult compromises to make a vehicle behave well on the street and track, after all - you can often be cheaper overall to have a daily driver and a track car/bike, and make fewer compromises. Also spend less of your time changing tires :)
Saying "well having a vehicle you trailer to the event is cheaper" assumes a baseline amount of money being spent that is more than most people have to dedicate to the hobby. That cuts out the entire low end of the hobby.
The lowest end of the hobby will change, obviously. Not convinced it will completely go away. I think my point was it isn't a crazy bump in expense to have a vehicle you play with only on the track, though it may not be the same vehicle.
And yes, it means you can't easily drive your hobby car out to a drag strip to see what it will do. Or a bunch of other things. But it doesn't mean there is no hobby to be had - at least I hope so.
4x4 is interesting, i hadn't considered but suspect that will be under more pressure than track days.
Owning vehicle you have to trailer to the track is kind of like owning guns and using them to hunt in England. Yeah you can do it but all the red tape and cost results in practically only the landed gentry being able to justify such a costly hobby.
That's really not the same.