I saw Sabbath at Download last year, and they were a shining example of knowing when it is time to stop - except for them, it has come too late.
John Lennon was asked throughout the seventies if The Beatles would ever re-form, and his answer was always the same - "It wouldn't be The Beatles, it would be four old guys who used to be The Beatles.'
And that is esserntianlyy what Sabbath have become - three old guys in their case, and it was purely an act of devotion from the faithful, rather than an innovative and massively influential band sealing the legacy before stopping.
Never mind - those not old enough to know where Metallica, Slipknot, and the entire heavy metal genre started from, have had a chance to see the people who created something that has endured forever, but they really should have stopped a long time before now.