I always felt  the same, that it would be far less worldwide environment pollution if we just kept our old cars & pickups longer. Yes there not the cleanest burning, but by the time we go thru all the different steps turning old into new.
It's more about $ than anything else.
The last 12 yrs we are required to have our septic pumped every 3 to 4 yrs. So it gets dumped into local farm cow poop pits and spread on top of the fields later around our little town. It washes out of the fields in heavy rains (soap suds and all) into our wells. So we all need to have a UV light to cleanup our water. Poop pumping is a big business around here.

I never really ever got the thing the government did to VW. I probably don't have it quite right but apparently VW  changed the computer when the car idled so it would pass emissions. Under normal run conditions it ran different and polluted more. The car got 50 miles to a gallon! If they really cared about the environment you would think they would want more 50/gal. cars on the road, not put them out of business. I agree with a lot that guy had to say. I have a buddy that drives and he says they are always having trouble with their trucks because of stuff added that guy talked about.

The Ford 6.0 is actually a great engine once you fix it.  People tend to be scared of them so you can usually get one cheap.  That generation truck is well built, has a great trans (5R110), and I think is quite comfortable even on long trips.  The engine made 325/570 stock.  My stock mileage was like 12/16.  I deleted my EGR cooler, replaced the oil cooler (after thoroughly flushing the coolant and replacing it with a CAT EC-1 rated coolant), added a coolant filter, installed a CAI, installed a better exhaust sans cat, removed the factory fuel pump and installed a FASS system, converted the fuel system to return side regulation, replaced the head gaskets & installed ARP studs, installed 175/30 injectors, installed a Stage 2 Powermax turbo from KC Turbo, and of course had a good tune written by Innovative Diesel.  Power is now more like 660/950 and mileage is 14/18.  1/4 mile goes by in 13.0 at 102.  Working on some short time improvements now that should put me in the mid 12s. 

A lot of smoke from a Diesel is either a problem or bad tuning.  I can control smoke with my right foot.  That black smoke, though it seems ominous, really isn't much in the way of pollution.  What you're seeing is bound carbon, which is nonreactive.  It looks bad, but its actually a lot better for the environment that the stuff you don't see like oxides of nitrogen, oxides of sulfur, C02, COO, and O3.  The reason they use particulate filters is because of a perception of pollution. 

I had a friend at church who had one of the VW's covered by the lawsuit.  My advice to him was to do nothing.  The ones that were "fixed" got worse mileage, or they would buy it back.  He did the buyback.  My guess is if you have one with stock tuning its probably worth more to those in the know. 

There are all kinds of small, efficient Diesels sold all over the world, but we don't get them here.  There are literally dozens of vehicles that get between 60 and 100mpg sold to the rest of the world.  The cover story is they don't meet emissions or crash standards.  Dig into it and that just doesn't wash. 

If all they are testing is opacity then the cat does nothing.  The cat it to reduce invisible constituents of exhaust like hydrocarbons.  An opacity test is just measuring soot output.  2002 OEMs were not required to install catalytic converters on Diesels, though Dodge and I guess Ford also started a year or more early on some models.  EGR started of Diesels in 2003.  DPF started in I believe 2008.  DEF is based on what the OEM had to do to meet emissions.  I know Ford started it in 2011 with the introduction of the 6.7.  DEF is not actually a requirement, but the engine has to meet the emissions standards and tight as they are a lot of OEMs need it to meet the standard. 

BUT, the thing about emissions is they are only Federally regulated when the vehicle is produced.  The Fed doesn't have the budget to set up testing in all 50 States, so they delegate that to the States.  States then determine what, if any testing program they will use.  Some States like CA are very strict, others have no testing for any vehicle whatsoever.  Among testing States, some do a drive test, others simply plug into the OBD2 port and if there are no codes and it shows the system hasn't been recently reset; you pass.  In DE there is no testing on Diesels, and no testing on anything over 8,600lbs GVWR, so my truck is doubly exempt.  They check the lights, brakes, and make sure the windows aren't overly tinted and that's it.