The very short version of my thesis on sci/tech change is that we have exponential increases in resources devoted to science and technology as a civilization, linear returns on many scales such as life expectancy, mean IQ, log GDP (which is still of mildly diminishing utility), etc.

Low hanging fruit depletion, the standard explanation for this, is very insufficient to produce the observed effect. Many other plausible effects have been proposed that could contribute to reduced scientific progress, including but not limited to excessive time in grant-writing and existentialism, various factors increasing conformity and selection for conformity locally and globally, and increased (until recently) environmental toxicity. Dysgenics may be a minor factor, decreased variance of all sorts is almost certainly more important, as is degradation in educational standards and institutions. I suspect that economic effects that I don't have time to discuss may be more important.