not to mention sampling, if you even want to stretch it that far) and it's certainly fair to say that Sly's music
Yes, definitely among the most sampled artists of all time, but it's not like Black Sabbath haven't been sampled too, although not to the Family Stone's degree.
Kanye West, Fugees, Busta Rhymes, Fatboy Slim, Beck, Insane Clown Posse, Enigma, Red Hot Chili Peppers, Beatnuts, Jungle Brothers, Alice In Chains, Ice-T, Beastie Boys, Butthole Surfers, Ultramagnetic MC's, Staind, Eminem, OutKast, Cypress Hill, Young MC, Vanilla Ice, etc., etc., have all sampled their work.
Their longer career resulted in a greater total number in album sales, simply because they had a far greater number of albums to sell (by about a three to one ratio)
Sly vs Sabbath is a case-by-case basis because there are all sorts of other variables.
A long(er) career does not equate great success. Also, you can have a short career (like ABBA's and The Beatles') and still trump virtually everyone else in popularity. You can be music-changing, decades-experiencing as Miles Davis and still not be able to be a cash cow like Elton John.
Look at the long career of Nana Mouskouri. Most Americans wouldn't know who she was and yet she was a huge seller around the world.