Agree about zone loading. Even better if you also load from the back as the locos do, but also KLM at AMS through airbridges. A lot of 753s are used on longer routes likes West Coast to Hawaii where loading times are less important.
The DC8 60s series were good aircraft from a cost viewpoint. The obvious way forward was to replace four engines with two more efficient ones, which gives you, err, the 753. There's a definite comfort gain in having twin aisles though.