As the author says, cool started gaining popularity in the 30s, as mass broadcast media became dominant and service jobs made up over 50% of the US work force [1]. Almost all service jobs, to a greater degree than farm and manufacturing, require the participants to signal competence. The cool persona is Hollywood's and Madison Avenue's counter-signaling answer to this demand. Cool is on another plane from those other, gaudy service sector workers. Cool doesn't flaunt competence, but if you give cool a guitar, gun, motorcycle, or skateboard, flawless execution will ensue. Anyone can adopt the mannerisms of cool (to Madison Avenue's benefit [2]).
What replaced cool? Genuine enthusiasm. With the rise of the internet, social media, and Youtube, entertainers are no longer selected by service-economy executives. Now entertainers rise up through skill, passion, and endless hours of self-promotion. Now artists and athletes have access to their fans, and their fans see their dedication and associate it with status. To get a sense of this change, watch a few early 2000s Scarlett Johansson interviews, then watch a few current Jenifer Lawremce interviews. The new cool is engaged and open, a tightly-packaged version of the qualities of Youtube star or professional athlete.
[0] https://en.wikipedia.org/wiki/David_Skinner_(journalist)
[1] https://www.minnpost.com/sites/default/files/images/articles...
[2] This chart https://www.ahrq.gov/sites/default/files/wysiwyg/professiona... from this 2018 study https://www.ahrq.gov/professionals/education/curriculum-tool...