You are right Dandy Don. Although the 707 had some advantages over its DC competitor, the 707 left the scene much earlier.
I can think of two factors that haven't been brought up yet.
The 707 could not be stretched as the DC-8 was, because the 707 was closer to the ground, and a stretched 707 would have experienced taiil strikes on take off. So I've read several times.
But even more importantly, the 707 was somewhat sacrificed in favor of the 747. It seems almost incredible now, when we have such a range of jets, but in the early seventies, all the glamour 707 customers, and some DC-8 customers too, went for the 747. For some of them, like PAA, it was too much and too early, and it was the beginning of the end.
If Boeing and PAA hadn't gone ahead with the 747, the 707 would have kept selling longer, stretched or not.
Now my silly question, since the 707 - 727 - 737 - 757 evolution was so smooth, is there any part, even trivial, of the 757 or 737NG, that was the same part of the 707?
Another thought, if Douglas had kept along with Boeing, there would be a DC-14 today!
I've flown thousands of miles and I can tell you it's a lot safer than crossing the street!