Fleetwood Mac's transition from full-on blues to Pop was quite a 180 degree turn, unprecedented. The original members were almost entirely formed out of old John Mayall groups (a kind of school for British blues...Green was Clapton's replacement). Old Fleetwood Mac produced such classics as "Black Magic Woman" and "Albatross". I believe the unsung hero of early Mac (which weirdly disintegrated) was the late Danny Kirwan, who left after the 1972 "Bare Trees" (a classic). I guess what happened then and after is a tribute to the steady managerial skills of the McVies and Mick Fleetwood, as somehow they survived and added the successful Buckingham/Nicks pairing. Like you, I kind of checked out on them after 1974 thinking they'd gone too "AM", but as you say some historically good stuff. Tragic that the talented early members couldn't hold it together, but I suppose their Blues genre may have had limited commercial appeal in the long run.