The "mid-list author" disappeared in the '90s.
True.
Earlier in the 20th century, publishers found it profitable enough to publish authors whose novels sold steadily but not spectacularly. The upheavals in the industry starting in the '70s changed that. In 1981, Thomas Whiteside wrote, “the economic threshold below which an author’s work is considered something that a company cannot afford to handle has risen.”
The only author of this group who had a smooth ride in the new publishing climate was Brooks, who wrote one bestseller after another.
Chesbro, who was equally prolific but whose sales were not as high, found that traditional publishers lost interest in him in the late '90s.
Mayer, who published his most recent novel in 1989, said that his past sales of 6 or 7 thousand were not high enough for publishers.
Hall, who had a good streak going, ceased to publish in the late '90s.
Jackson appears to be the last surviving midlist author: after bouncing around a while, he found a secure berth with a literary-minded traditional publisher.
Other novelists have turned to nonfiction (Lowry, Nickerson), or moved to small publishers (Ducker, Bredes), or turned to new storytelling media (Swigart).