I think the 707 and DC-8 originally were quite comparible in economics and toughness, but the situation changed from 1981 onwards. In the economic downturn and high gaz prices then, both noisy types were almost worthless and the majors wanted to get rid of them as soon as possible. In case of the 707, the USAF
made good use of the airlines parking them to buy the best ones up, use engines and other parts for their KC
-135s and then scrap them.
The best and newest DC-8s, the 60s series, were adopted in a reengine program which made the 70s as economical as any 70s and 80s airplane type again so this explains why these are still around.
The 707 won as a passenger carrying aircraft, while the DC-8s passenger days are already over for 10 years. The DC-8 is still a frontline freighter though.
But then if you take the military variants of the 707, the E-8s and AWACSes and VIP aircraft in account, there are around more then you think (and then I don't mention the KC
-135 variant which is a different aircraft, even while the youngest frame is 40 years old, 2/3rds are still in service!).