Look, I am 100% for the conclusion that we should not as a default case have people working too much! Even a 40 hour work week, for work that is statistically usual today, is in my opinion inhumane.
That philosophical belief however does not win out in the context of this particular analysis.
My agenda is to support sensible discourse on the question of how our brains adapt and how we live, using rigorous thought. Maybe I'm not seeing the problem with my reasoning but it seems pretty clear to me.
The problem is that you automatically get an upward or downward parabola, plotting cognitive scores versus working hours, if you have a non-zero coefficient in working hours^2
It's very unlikely that you'll get an upward parabola, especially given the strong anticorrelation between unemployment and cognitive scores that they use.
In science, you have a judgment call for which predictors you plug into the statistical analysis. Choosing any particular function, be it working hours^2, sin(working hours/100), or even, say, ballmerpeak(alcohol content) will effect the statistical results in factor analysis or anything else.
Since we chose only linear and quadratic functions to get coefficients for in our statistical analysis, the functions are going to be a parabola -- either up or down.