Diesels were already well on the way to being much cleaner than ever prior to this nonsense. Better designs combined with computer controlling were allowing some very fuel efficient and clean designs. However the powers that be were still living in the naturally aspirated diesel days, and NOx was targeted first, by introducing EGR, by doing so, the already efficient motors started generating larger amounts of particulates from unburned fuel. So, then we had to introduce Diesel Particulate filters to capture the soot. If they had gone with SCR utilizing DEF fluid out of the gate instead of EGR, it might have been possible to eliminate the lion's share of the problems we have now. But even with SCR now, once these folks decide something has to be in place, it is almost impossible to get them to change their minds.
The stacks on my pre-EGR, no emission control motor never have nasty soot buildup on the stacks. My 2006 Cummins ISX EGR equipped one, the soot buildup on the stacks was terrible. What changed? Just the advent of EGR. Also, the non EGR equipped motor got, and still gets, better mpg than the industry average for OTR trucks. It was a fight to get the EGR motor tolerable mpg. EGR cost an average 1-2 mpg loss on commercial class 8 OTR trucks across the board. That doesn't sound like much to the average user, but for a truck that runs 120K to 140K miles a year, we are talking about well over $10,000.00 in wasted fuel because of EGR alone. The general formula is for every 1/10th drop in average fuel economy, you lose an average of just under $1000 on that one truck in one year. And you multiply that over the 2.5 million commercial trucks in the U.S. alone and the numbers really make it hard to justify EGR.
Things are coming around, and class 8 truck motors are getting some pretty good mpg now, but the emission control systems and the advanced designs to get there is adding roughly $20K - $25K to the cost of a truck. And the operational cost and down time when these systems go goofy still is hurting everyone.