I agree that intensity needs to be high, and that going to failure is the assurance of consistently high intensity. However, we are talking about total sets per muscle group per week here. I'm not sure that Jones and Darden necessarily endorsed a single set a week, at least for the larger muscle groups, although I know that Jones was whittling the numbers lower and lower as time went on. And while Darden embraced once weekly workouts for a time some years ago, he seems to have backed off this view since then, both because he trained you twice a week, as I recall, and because he has since been interviewed where he supported a not-to-failure workout during the week for "active recovery." (I never quite understood that term.) Since he trained you personally, perhaps you could shed some additional light on this very subject. I'd appreciate it.
And if we're really going to take Occam's razor to the matter of overall workout volume, you will recall that I posted a couple of studies a while back that concluded that single joint exercises don't add to either strength or size to a muscle group when multi-joint exercises have already been done addressing that muscle group. This applied to trained men in the study who either trained naturally or took steroids: https://www.researchgate.net/public...pometric_changes_in_recreational_bodybuilders And it even applied to untrained women: https://www.researchgate.net/public...nce_Training_Program_in_Untrained_Young_Women I know that both Jones and Darden prescribed single-joint exercises. If we can accept the studies at face, then they needn't have bothered, thereby lowering the minimum total workout volume further still.