I think the main thing about 20GG was that it started a bit of a Shads renaissance... a decade or so after they had last been truly successful. It kick started the touring again and brought the Shadows back into the public gaze... in a way that even the Eurovision hadn't really. If you type The Shadows 20 Golden Greats into a certain well known internet auction site, you get about 500 results on average, This, to me, suggests that there were a heck of a lot of people who bought it in 1977 - sadly, many of them possibly no longer with us. I'll speculate that without 20 Golden Greats, the Shadows wouldn't have lasted until 1990.
Interestingly, when you type 'The Shadows Greatest Hits' into the same auction site, you currently get more than 1,000 results... but many of them are for other albums as well as the primary one. I think String of Hits is, not surprisingly, equally as popular as 20GGr.
P.S. Been meaning to mention this for a while... Has anyone else ever noticed that the cover design of Thank You Very Much contains obvious visual nods to both 20 Golden Greats and Cliff's 40 Golden Greats albums? Namely, Cliff's head in profile and looking right and the shadows (silhouettes) of the guitar necks and headstocks. Not a coincidence I feel.