You can find instructions on how to make DEF simulators using raspi's to fool the ECU into think the DEF system is working, people who do care about emissions will still carry these for emergency so they continue on their trip and get to a shop later. The derate was over zealous for sure and was a bad policy.
Also DPF is a performance issue since it blocks the exhaust to some degree, same with catalytic converter with def nozzle so no its not just EGR at all. DPF also consumes more fuel for regens.
2027 diesel regulations was to mandate even more NOx control but also specified manufacturers were required to have 100,000 mile 10 year warranty on emissions systems, its 5 year, 50k now. I believe thats dead in the water now.
A diesel engine with a deleted SCR system puts out 40 times the NOx of a working one. Thats 40 trucks going down the road to 1 equivalent. NOx causes asthma and acid rain, its not for the environment as much as for you directly.