Ultimately, I'd agree that emissions should be on a "per mile" basis ONLY, without regard to engine size or type. It may take a little while for all of the tractor and other large diesel manufacturers to figure things out, but they will. Heck, Otto-cycle engines are now at the peak of power in history and the cleanest.
I will note that my Mahindra has no DPF, but uses a catalyst only. So far, no issues, but only in the low 100 hour range.
Don’t worry to much DOC’s fail too. It’s very similar to a catalytic converters in your gas powered vehicles.