The tractor industry in the US is not a trillion dollar industry nor was the investment into the DEF solution. The North American farm machinery market size is under $16B per year, total, (re: statista.com) some of which is tractors. Not to say that is small, it is just not trillions. But I get your point. The industry does unfortunately have to get re-paid for the EPA's arbitrary rulings that dictate manufacture. I suggest the following:
1) The EPA has been generally out of control, answering to no one and far more responsive to activist zealots than to real workers of ANY industry.
2) I would bet money that the total air pollution originating with farm tractors is less than 1% of what it is for highway trucks. An engineering guess, probably far less than that. Searching but have not found the stats. I will. This hideous lack of prioritization SHOULD be putting the EPA on report, not the Ag industry.
3) Just as with the attempted destruction of VW for having outsmarted the EPA, the out of control regulators know they have a stranglehold on all industries that costs more to fight in court than it costs to attempt to comply.
4) It is extremely likely that farm tractors contribute such a tiny fraction of overall air pollution that any common sense in EPA (...an oxymoron of outlandish proportions) would have placed farm tractor engine regulation so far down the priority list it would never see the light of day. But the EPA has no such prioritization. They are the loosest of loose cannon.
With that off my chest, what is the better solution ??