Interesting that since the end of Vietnam the number of non-science PhDs has been relatively stable. I was under the impression that those had increased, too. The graph of the science PhDs reminds me so much of Player Piano, Vonnegut's first novel (I think).
I would hypothesize that remuneration of technology/engineering Ph.Ds outside of academia kicks the shit out of the remuneration of science Ph.Ds. Not only that, the number of firms requiring Ph.Ds for their c-suites has contracted through consolidation. Academia, on the other hand, has erupted: The whole point of tenure is you don't have to teach as much anymore, right? So what are you going to do with your free time to justify your existence? Peer review works. Note that I have less education than almost all of my friends. I have less education than my parents. I have less education than my relatives. I evaluated early and often that an advanced degree was not going to serve me. But then, I also knew that if I couldn't support myself 100% the minute I matriculated I was utterly and totally fucked. Half the population of the United States doesn't support themselves anyway so they have a different perspective.