Actually the new DD15 and DD16 have the longest estimate of longevity at 1.2 million miles. Not only are they duplicating the great fuel mileage the Series 60's got before 1998, they are running so clean that for over the road applications, the recommended oil change
My post was not about the lifecyles of the actual engine. I was referring to the lifecycle of the engine design. Ford has gone from the 7.3 to the 6.0 to the 6.4 to the 6.7 mostly due to the emissions issues. We don't really know if the 6.4 could have met 2010 emissions with changes. Ford decided to cut Navistar out of the loop on the 6.7.
Ever tighter emissions standards mean engine designs don't stick around as long as in the past. The Series 60 had a pretty good life, but it is was replaced by the DD15 when emissions standards tightened. It may be that the emissions change was simply a good time for a new engine rollout for Detroit.