The change can be traced back further, to judge Richard Posner and "Natural Monopoly and its Regulation" (1968) (http://chicagounbound.uchicago.edu/cgi/viewcontent.cgi?artic... (PDF)), subsequently published by the anti-regulation, pro-monopoly Cato Institute, and earlier, judge Robert Bork, dating to earlier. In the 1960s, see:
"‘Antitrust was defined by Robert Bork. I cannot overstate his influence.’'
This reflects earlier treatment of monopoly within Libertarian economics texts for popular consumption, notably Harry Hazlitt's Economics in One Lesson, which addresses monopoly by ... dispensing with it virtually entirely:
Barak Y. Orbach, "THE ANTITRUST CONSUMER WELFARE PARADOX":
“Consumer welfare” is the only articulated goal of antitrust law in the United States. It became the governing standard following the 1978 publication of Robert Bork's The Antitrust Paradox. The consumer welfare standard has been instrumental to the implementation and enforcement of antitrust laws. Courts believe they understand this standard, although they do not bother to analyze it. Scholars hold various views about the desirable interpretations of the standard and they selectively use random judicial statements to substantiate opposite views. This article introduces the antitrust consumer welfare paradox: it shows that, under all present interpretations of the term “consumer welfare,” there are several sets of circumstances in which the application of antitrust laws may hurt consumers and reduce total social welfare. This article shows that, when Bork used the term “consumer welfare,” he obscured basic concepts in economics....
"‘Antitrust was defined by Robert Bork. I cannot overstate his influence.’'
https://www.washingtonpost.com/news/wonk/wp/2012/12/20/antit...
https://www.cato.org/policy-report/julyaugust-1999/cato-book...
Bork's "landmark" treatise on monopoly remains curiously unavailable at many libraries:
https://www.worldcat.org/title/antitrust-paradox-a-policy-at...
This reflects earlier treatment of monopoly within Libertarian economics texts for popular consumption, notably Harry Hazlitt's Economics in One Lesson, which addresses monopoly by ... dispensing with it virtually entirely:
https://fee.org/resources/economics-in-one-lesson/
Contrast Alfred Marshall, Principles, 8th ed, leading collegiate text at the time, with a 15 page chapter and a multitude of mentions.
https://archive.org/details/in.ernet.dli.2015.149776/page/n4...
Barak Y. Orbach, "THE ANTITRUST CONSUMER WELFARE PARADOX":
“Consumer welfare” is the only articulated goal of antitrust law in the United States. It became the governing standard following the 1978 publication of Robert Bork's The Antitrust Paradox. The consumer welfare standard has been instrumental to the implementation and enforcement of antitrust laws. Courts believe they understand this standard, although they do not bother to analyze it. Scholars hold various views about the desirable interpretations of the standard and they selectively use random judicial statements to substantiate opposite views. This article introduces the antitrust consumer welfare paradox: it shows that, under all present interpretations of the term “consumer welfare,” there are several sets of circumstances in which the application of antitrust laws may hurt consumers and reduce total social welfare. This article shows that, when Bork used the term “consumer welfare,” he obscured basic concepts in economics....
https://academic.oup.com/jcle/article-abstract/7/1/133/75097...