I’m not sure how you could get the free market to regulate NOx emissions via market dynamics. Soot, maybe, as soot generation is a sign of incomplete fuel consumption. But NOx is the nitrogen fixation that occurs at the combustion temperatures in the cylinder, and it is nasty stuff that can build up in local air masses, and it also causes acidic rain (the planet produces its own NOx via lightning, but diesels could easily top that).
NOx is also not very pleasant to breathe in traffic.
If there is a market force that can control it, I’m all game for it, but I honestly don’t know it.
I would see cars coming with ventilation filters that filter out pollutants that otherwise would entire the cabin to handle traffic emissions, before the industry self-regulated NOx output without government intervention.
Maybe I just don’t see the solution, and someone else does though.
simple, people buy what there is demand. If there is demand for NOx less diesel, then there will be a market for it.