As someone who drifts in and out of PM roles, this. The very nature of Product Management is quite squishy in practice. I don't have a scientific number on this, but in my experience out of a group of 10 PM's, there are 1-2 great ones, 2-3 decent ones who will likely be great if they keep at it, and the balance are people who want to work in "tech" but only have soft skills.

The commonality in the best PM's I know is having more than a passing knowledge of the domain they work in.