Of course it is much easier to manage things that you can measure.
That does NOT mean that if you cannot quantitatively measure it, that it can't be managed
It certainly does not mean that if you haven't yet developed an effective metric, that it does not matter -- it's just mind-blowing that McNamara went that far.
The way to do it is to FIRST figure out what matters.
Then figure out if it can be measured, and if not, how to qualitatively assess it. Then also figure out how to ensure that the metrics don't become the goal (with perhaps the sole exception being if you are in sports and the goal is winning races/games, so that's a pretty good metric in a suite of metrics).