Basically, the idea of various SS classes was to conduct a defense in depth against NATO navies, primarily carrier task forces or, in earlier scenarios, amphibius forces. Zulus (just 26) and Foxtrots for long range recon and effect, Whiskeys (236) and Quebecs (100) for closer in defense. One has to remember, that even Whiskeys had just some 8500nm range surfaced, at 10kts speed. Snorting speed 7kts. It's endurance was 30 days. Operating from Soviet Kola bases, that's not enough to conduct offensive operations in the North Atlantic, as even getting through GIUK gap would take some 1400nm's or 9 days, leaving just some 11 days for operations.
Later on, with bastion strategy enabled by Delta -class submarines from 1970's onwards, the defense of bastions became a priority.
Whiskey was not based on XXI class, Zulu did incorporate a larger amount of XXI ideas but had various problems.