To believe that early zoom lens designs of the sixties had and optical quality comparable to today's is ridiculous.  This ignores the progress on optical design of the past few decades. Early zoom designs typically consisted of three lens groups (convergent,divergent,convergent); the 2d group was fixed, and the 1st and 3rd groups were mobile.  Furthermore, the 1st and 3rd group were rigidly linked, so that a complex and difficult to manufacture non-linear cam-based group distance regulation mechanism wasn't needed.  The downside of the rigid linking was that such a design can limit optical aberrations only over a very limited focal length range.
The development of sophisticated complex non-linearly driven four-group based designs (focusing, variator, compensator, relay master) is a fairly recent phenomenon that has been enabled by the development of new optical design techniques, computation algorithms, manufacturing — e.g. lower cost aspherical lenses, numerical control machines to manufacture precise non-linear driving cams — and material science — e.g. glasses with anomalous dispersion characteristics.