The automakers have reached the limits of technology in cleaning the diesel emissions, at least with the diesel fuel available in the USA.
I think you have made the wrong conclusion.
There *is* existing technology out there that allows diesels to burn cleaner, by using catalytic conversion in the exhaust with DEF. Unfortunately, this specific technology it has a cost and a hassle factor (DEF is a consumable, much more so than the platinum in a gasoline catalytic converter), so it is rarely deployed in consumer light-duty vehicles. Maybe someday, someone will come up a more economical technology that has less hassle factor for catalyzing exhaust from diesel engines, but to assume we have reached the limits of technology is an unwarranted conclusion.
You are likely to have been confused by the high-sulfur problem which has historically plagued diesel emissions in the US. This particular scandal was related to NOx emissions which is fixable (albeit inconveniently) by using DEF technology.