The survival of DC-8s had more to do with their being re-engined with CFM56s than any structural superiority over the 707. A few CFM56-engined 707s were eventually built for the military, but none were retrofitted.
The DC-8 also had the advantage of being stretched, the 707 gear was too short to allow for this.
A more pertinent comparison would be how many DC-8s with
original engines are still active versus the 707. And I'll wager the last DC-9 retires long before the last 737-200.
Edited to say that, having remembered Airborne's large fleet of cargo DC-9s, I am now not so sure about my previous statement
.