The question was whether education is a good investment, which poses the idea of some kind of return on an initial cost. The investment question was posed in the form of personal cost and personal return. In that sense, productivity is not a measure of the personal return of an education. I mean a few people might see it that way, and it could be associated indirectly with success, but by and large productivity on it’s own is not a benefit.

Productivity could be seen as the social return for a social (tax based) investment in education, and indeed we do measure things like GDP and compare nations on a global scale. In that sense, education appears to be a good investment socially. Countries that have high rates of education also have high GDP.

Aside from that, unfortunately, it’s extremely difficult to measure “productivity”. You might want to take a stab at trying to define what that even means for people in various careers. It’s easy to measure the productivity of factory workers, but insanely hard to measure productivity for doctors or writers or advertisers or nearly any white collar job. Is productivity measured by how much output there is, regardless of quality? Is productivity measured by how much money exchanges hands? These things could be even worse than valuing education, no?