Re: Today VW ...
The real problem they have is that these systems need to be properly looked after. They wear out, run out of urea, get fouled up. A diesel car when it's brand new is pretty clean, but after a 100,000 miles who knows? Especially if you run it on cheap diesel.
CARB has made it quite difficult for the car owner to cheat on this one. If the urea tank is empty, the vehicle will warn you a few times politely and then will refuse to start the engine. They call it "inducement action". Also if some wise guy fills in water (or any other fluid) instead of urea, the system will detect it and politely gives the driver a wedgie. As for long term emission stability, CARB forces the vehicle makers to do the homologation with aged components in the first place. In the second place they regularly check aged vehicles from the "real world", just in case. As for the bad fuel, OBD asks for quite strict emission diagnostics. You have NOx and particulate mass sensors that will find out about if the emissions drift too far out of the window.
They are quite clever chaps and take into account that vehicles are sold to normal people that might want to "cut expenses".
What they didn't account for was active cheating from the manufacturer's side.
I suppose that will change in the future,,,