So, I found a Nader Elhefnawy blog post on the shuttering of Harlequin’s Gold Eagle imprint for “men’s adventure” cheap thrillers in 2015 (although Harlequin has continued to release new Mack Bolan ebooks since then). Besides the increasing diversity in media as a whole, the genre is mentioned in the post as being squeezed both from above (from bigger-market, less assembly line-ish cheap thrillers) and below (from independent/self-published ones).
Now the indies and the big-timers both have structural weaknesses and strengths. As for how the Gold Eagle Bolans (and similar professional assembly line fiction) held up, I’ll have to read them. Even Ahern’s Survivalist doesn’t really match up, as that was a giant serial made by one person, not 27 standalone books made by different ones.