I just don’t think that’s supported by the data though. On virtually every measure, living conditions have improved across the globe over the last century. You can pick out specific metrics where things are worse in specific countries (housing is the most obvious example where affordability has declined), but taken as a whole I just don’t see how you can draw any other conclusion.
There are probably ways to correlate the military test with a standardized IQ test, and which point the military test might be a rough proxy for IQ. If that was the case, the 80 IQ rule might be roughly accurate.
I don’t know if that’s been done though. Just playing devil’s advocate.