Specifically, reducing NOx (without using DEF) pretty much requires the engine to burn a richer mixture. This means more fuel injected, so less fuel efficiency

Specifically, reducing NOx (without using DEF) pretty much requires the engine to burn a richer mixture. This means more fuel injected, so less fuel efficiency and more unburnt hydrocarbons clogging up the particulate filter. Without retrofitting DEF systems onto the older cars, any fix is likely to be a significant hit to efficiency and reliability. I think maybe performance would suffer too but I forget how that would work.