I think it's not a source but a removal of distortions. If there are a lot of people with PhDs but what society really needs is a few ditch diggers, then it's right that ditch diggers be highly paid. If theoretical research is "difficult" but fun and rewarding and ditch digging is "easy" but boring and stigmatized, it's right that ditch diggers be paid more than researchers - and note that this will also mean more effort is devoted to automating ditch digging than automating research.
I don't think that the same number of people will still feel motivated to acquire a PhD if they know that in any event ditch digging will get you more money to begin with.
If people don't want to get a PhD, and we don't have much need for people with PhDs, then reducing the number of people who get a PhD sounds like mission accomplished.
