The thing is, that really isn't going to save them from litigation by the gov't should the EPA decide to go after them, or IF they are even able to. Because the "off road use only" clause doesn't work, because so long as the car was originally designed, spec'd, and sold for road use, then that's what it has to meet.
Therein lies the rub: the racing community was all up in arms over this, because if you really applied that rule, someone building a race car that started as a road car would have to keep all the equipment intact. Which of course, nobody does. I'm talking ALL motor sports with production-based vehicles/engines. Truck pulls, dirt track racers, drag strip racers... anything that was originally a road vehicle. Obviously this is an impossible rule to enforce, and it probably is hoped that the fear of consequences will dissuade the tuners from doing it. But so long as the draconian regulations force endlessly fragile and expensive equipment on to vehicles and machinery, the outcome is always going to be people trying to modify it.
When TDIs first came along, all we really had to deal with was the EGR system... and with the old 500ppm "low sulfur" fuel, the EGR was problematic, but it was fairly easy to deal with. Either you did a quick, simple, cheap, widely available software change (early ones this meant changing the PROMs in the ECU, hence the term "chipping"), or you just dealt with periodic intake cleaning. Neither was much of a nuisance in the grand scheme of things. You still enjoyed a car that had good driveability, unrivaled fuel economy, and whichever route you took to deal with the EGR was relatively easy and cheap. And even with EGR, we still had a car that was FAR simpler than ANY of its competitors of the time for engine management. The VE TDI is a dirt-simple engine. Its electronics are super simple, it is still a mechanical based fuel injection system that in 1996 was already two decades old in Volkswagens. Tried and true. Simple, tough, reliable. Then when ULSD eventually came along, the EGR issues largely went away, or at least lessened to a point that it is hardly worth mentioning.
But it has evolved and become simply too much to deal with. Volkswagen's answer was to try and cheat the system, they got caught, then got all butt-hurt and took their toys away. And it is sadly becoming clearer every day that the only realistic way to deal with these modern diesels in many instances is to do extensive modifications to them just to make them useable long term. I see 6.7L Ford trucks every day with sooty tailpipes, so you know those are deleted... because stock ones don't do that. I'm talking the ones with the double fluted pipes, not some brodozer guy that has a giant soccer ball sized pipe sticking out the back and a lifted suspension and rolling on Chinaspokes wrapped in no-name Chinapops displaying to the rest of the world just how small and useless his genitalia is. I'm talking about company trucks, vocational beds and boxes, livery on the side, that they have chosen to delete because sometimes they just need their damn equipment to WORK.
Of course, it also doesn't help (regarding the domestic pickups) that the diesel engine game has basically become a dick swinging contest to see just how much frame-twisting, slushbox-munching (because they're ALL automatics now), axle-punishing tire-smoking horsepower and torque they can squeeze out of these things. Does anyone really need an engine that belts out 1200 torque just to drive around mostly empty most of the time??? Because if you do, Ford will sell you one.... can't do much for your teeny peeny, though.