When the head of Audi, became CEO of VW, the goal (pressure was on) to increase US sales three fold. The CEO envisioned the diesel, playing an important role. At the time Mercedes, & VW had a licensing agreement for a Mercedes diesel, also VW had the in house (VW) E189 TDI engine. The decision was made to go with E189 & drop the agreement with Mercedes. Two super star engineers, warned, but the pressure was on to triple US sales.
VW was hell bent on being the number one mfg. surpassing both GM & Toyota. Greed got in the way, it cost the CEO his job, & exposed, a cobweb, of problems, at VW & the industry in general. Now exposure, promises to improve the situation.
True VW made mistakes & was caught, yes they will be doing damage control for awhile
This is not an isolated case, cheating is "ROUTINE" in the industry, EPA (due to factors previously mentioned), relies on mfg. to Self Test, then report, results to them. This CAUGHT could have been any mfg. Greed or the lust to be #1, made it more probable, that cheating would be uncovered.
More than (you suggest, "All they had to do...") changing DEF's, cat,DPF, or EGR. It's just not that simple, or what you describe was well known by those two engineers. What did get in the way, was the CEO's desire, to be #1, or GREED
This is exactly why, we need regulations/regulators, who can perform there job, with little interference, from politicians with an agenda. We also need whistle blowers, who can come fwd. without fear of retaliation. etc.
Regarding pollution, some pollutants are much more important than others, & hence the others can be tolerated to a greater degree. Any time you burn, fossil fuel, be it in a jet engine, or internal combustion engine. You get pollutants, & regulators (science), have to go after (decision) what is the worst, & how to attack, the problem, without, killing an industry. It's well known that going after a particular pollutant, CAN & DOES decrease efficiency, & can cause other problems. Knowing this informed decisions are made, biased (both ways) abound on the internet. Googling some aspect, then reading biased, or non biased paper, is not a GOOD substitute for, the science involved in keeping us pollutant free.
Mostly we agree.