There is a lot of things we can't measure that are worth doing. Like maybe improving your relationship with your wife and kids. Or reducing distraction and improving your focus while you are working.
If the only thing you did and mattered in your life was your diet then yes, you would have to keep measuring it obsessively.
But I don't want to spend my life focusing on my diet to exclusion of everything else. I have lots of other stuff I want to be doing. A healthy diet for me is one that allows me to not have to monitor and measure and weigh myself constantly.
Not measuring is the goal, not the fault of the method.
And what does that have to do with prostate cancer, I wonder?
Not everything that can be counted counts and not everything that counts can be counted. (attributed to Albert Einstein)