A major part of why the DC-8 hung on well into the 21st century (albeit almost entirely in cargo service) was the late-1970s/1980s conversion of 110 DC-8-60s (out of a total of 262) into DC-8-70s; the main (or, for the DC-8-6/72 and DC-8-6/73, the only) change was the replacement of the DC-8-60's old, loud, thirsty Pratt & Whitney JT3D low-bypass turbofans, and the nacelles thereof, with new, [relatively] quiet, efficient General Electric/SNECMA CFM56 high-bypass turbofans (and the nacelles thereof).1 2
Only the DC-8-60 was eligible, though; the short-bodied versions of the DC-8 (the DC-8-10/20/30/40/50), which formed a slight majority of all DC-8s ever produced (2943 out of a total of 556, as compared to the 262 DC-8-60s)5 needed not apply, even though they would have benefitted just as much from the reengine as the DC-8-60 did, and the process would have been mostly the same (in fact, the DC-8-61 was merely a straight fuselage stretch of the last short-bodied version, the DC-8-55, and, yet, the former version got [the opportunity for]6 a reengine, while the latter did not).
Why did the short-bodied versions of the DC-8 never get offered the CFM56 upgrade that was extended to the DC-8-60?
1: Thus continuing a long tradition of up-engining older-model DC-8s to newer-version standard, although the DC-8-70 was the only DC-8 version for which this was the only possible way of getting one (without any being built new).
2: The April 1979 decision to reengine DC-8-60s with the CFM56, incidentally, saved said engine from extinction, allowing it (in multiple later versions) to later see service on the 737-300-through-900, A320ceo, and A340-200/300.
3: Over half of these were of the latest and greatest short-bodied DC-8, the DC-8-50 (142 built new, plus a total of 20 converted to DC-8-50 standard from DC-8-10s [12 aircraft], -30s , and -40s ); there were also 52 DC-8-30s (not including the five converted into DC-8-50s), and smaller numbers of -20s and -40s (but not -10s, all of which [except for two which crashed first] were converted to DC-8-20 or DC-8-50 standard).4
4: I am aware that many of the short-bodied DC-8s had either crashed or been retired by the time the 1980s rolled around; however, this was also true of many of the almost-as-old DC-8-60s.
6: Surprisingly, many DC-8-60 owners and operators did not take the opportunity to upgrade their aircraft. Out of 262 DC-8-60s produced, only 110 were reengined into DC-8-70s; some of the remaining 152 DC-8-60s were presumably aircraft that had already crashed or been retired by the time the reengine programme came around, but, even of the DC-8-60s that were still around, many went unupgraded.