During a recent court case, a lawyer defending Volkswagen said that the company might not be able to fix all the cars involved in its emission rigging scandal in the US, or at least, not necessarily fast enough to be compliant with air quality regulations within the expected timeframe. This isn't simply because there are many affected vehicles, but also because VW may lack the technology to make the cars compliant without affecting too much their performance and fuel economy.
Volkswagen has not found a way to reduce emissions that satisfies federal and California regulators. In any case, the company would still face legal claims from owners if the cars’ performance suffered and the vehicles lost resale value as a result. It may be simpler just to buy cars back, or offer owners generous trade-ins on new Volkswagen vehicles. (source)
Air quality rules are stricter in the U.S. than in Europe, where VW might be able to more easily fix its non-conforming diesels.
VW is not only on the hook for cars that it has sold, but it also faces hundreds of lawsuits from car dealers who are stuck with diesel cars that have fallen in value and that are now harder to sell because of the emission rigging scandal.