Not that I agree with is basic premise, but diesel emissions post 2011ish are expensive.
DEF = Diesel Exhaust Fluid... Urea sprayed into the exhaust. Gets rid of Oxides of Nitrogen (NOX) that cause smog & other air quality or breathing issues.
DPF = Diesel Particulate Filter... A catalytic converter looking device that filters soot out of the exhaust then periodically burns off that soot. Prevents lung choking black exhaust.
EGR = Exhaust Gas Recirculation... a diverter that cools & sends some exhaust back into the intake to lower combustion temps. Helps minimize NOX that forms at higher temp combustion.
EGRs have been around a while, but the other 2 are relatively new (2011ish). Best case scenario if any of the fail you are looking at a grand, possibly several grand, or a new engine. With all the electronics monitoring things, you can't just leave them broken as you get put into limp mode. In the past year or so the EPA has killed the delete market where people would disable & remove the emissions gear.
Lots of parts to fail & they can be a bit temperamental. Basically diesel emissions gear is as crappy as gas emissions gear was in the 80 & early 90s. All of the big 3 truck OEMs have similar systems with similar issues.
Shortly after getting my 6.7 with 100k on it the DEF heater went out. Was about a grand to fix. DEF is mostly water, so it needs a heater & funky plumbing to drain things back into the DEF tank so it doesn't freeze in the lines. Same but different story with other emissions components.
The emissions gear is a reliability downgrade & hinders performance. But with modern diesels pushing 1,000 ft/lbs of torque I don't see that as a huge problem. A modern emissions strangled diesel smokes a tuned older diesel hands down. Except for the fact there isn't a bit if smoke when it does it.
As much as I hate the emissions crap on my truck (or Kubota) i like clean air more. Nice starting either in the barn with no smell, cloud or coughing fit, much less other long term pollution & smog related health issues.
Some people enjoy wrenching on their trucks & what's required to keep older machines on the road. Or at least make that tradeoff for a simpler cheaper machine. Personally I don't & like modern bells & whistles.