I've seen various posts where writers speculate that if VW goes heavy with fixes to the cars in question, that the majority of owners won't go for it. Owners won't accept possible reductions in fuel economy, performance, and reliability.
I am one such owner, who does not like the idea of any "fix" to my car, even though the fix for a 2015 TDI may not be very intrusive. I certainly would not be happy about something like adding an SCR system to my car (if I had an earlier model).
However, I don't think the typical car owner, even the typical owner of a TDI, is like me or the rest of the people on this board. The typical car owner just doesn't think about their car that much.
Should the parties involved feel that fixes to the cars in question are the way to go, I think dangling $$$ in front of the owners would get a pretty high rate of compliance.
Recall: We will be adding an SCR system to your car to reduce emissions. This may cause slight reductions in fuel economy and performance, and will require the addition of AdBlue at regular service intervals. Most drivers will still get fuel economy higher than published on the window sticker. We will warrant the system for X years and Y miles from the time of installation. To compensate you VW will write you a check for $2,000 upon completion of the recall. Or, VW will give you $3,000 toward the purchase of a new VW.
I'll bet that would get a pretty high rate of compliance. I think most TDI drivers are not paying nearly the attention to this situation that we are, and a $2,000 payout (maybe even a $1,000 payout) would get most of them to the nearest dealership for the recall. They cannot tell you how much peak torque the motor produces, they have no idea what a HPFP is, etc. They would see a check for $2,000 like a bluefish sees a Gotcha. (Hey, it's a really nice day out here and I wish I was out fishing. Perhaps I should have tried an analogy that involves goats?)