There's likely always going to be tension between how many people management thinks are necessary to do the work and the reality of people not working like mach

There's likely always going to be tension between how many people management thinks are necessary to do the work and the reality of people not working like machines. If the union workers think that management would like to cut things so much that it will cause the quality of the resulting work to suffer drastically, to say nothing of their health, then they're not arguing for inefficiency. And if they don't trust that any sort of metric for "good enough" wouldn't be gameable since the management has more expensive lawyers to write contracts than them, a blanket rule makes more logical sense than trying to bet you didn't leave a loophole. (All opinions my own, obviously.)