Well some of them (the DC8-70 rebuilds) had new CFM56 engines installed 20 years ago, but the even the later DC8 airframes are all now knocking on at 40 years old, and have had a far longer innings than most would have believed, They won't go on for ever.
This aircraft doesn't actually have a great deal more capability than a new executive A319CJ with half the number of equivalent engines. Bad news with $90/barrel oil.