The biggest mistake was Douglas scrapping the line (paid by the US Government).
Until the 747, the -63 with about 240 seats, out-lifted the 707-320 (184).
The DC8 could carry more, go further, and had a ACN of only 23 to 25 (lower than the 737-200 with low pressure tyres or a 757).
Thus the probems with Newark (and others) in it's later life with increased PCN requirements.
The "8" was a really great aircraft.