Great addition to the discussion. I’ve read Robert Gordon’s book (and written a few articles on it as well), and have ordered The Wealth of Humans, because I want to be sure I’m getting the full picture here.
In my reading of it, Gordon drives this argument hard in exactly the reverse. WWII was very nearly around the apex of total factor productivity growth, and this was clearly because an exogenous factor created a shortage of labor unlike any other time in the 20th century as well as a direct stimulus which was also inherently pro-technology due to the competitive nature of war.
In your arguments, the phenomenon of the decline of growth is actually self-supporting. Technological improvement produces a glut of cheap labor as a side-effect, diminishing the need to continue that technological advancement. This strikes me as a sort of Keynesian crisis with a novel exasperating factor.
The more labor-saving technology we develop, the less of a need there is for more of it. I could choose to interpret this as a simple demand-supply relationship, but the corresponding social problems are sure to cause various Nth order effects. It may actually be that technology is very ineffective at moving society forward of its own accord. Looking at the WWII example, it may be that an external stress on the system is needed to increase productivity while not simultaneously destroying the demand for further productivity gains.