If that was the cause, 1) book sales would drop overall, and 2) nonfiction books would perform the same as fiction books. The OP says neither are true:
"What has happened in the last ten years is that the overall industry has grown modestly, but fiction and nonfiction have basically flipped positions. In 2015, 6 of the top 25 bestselling hardcover books—this is a not-perfect but useful proxy for new books published—were fiction. In 2025, 18 of the top 25 hardcover books were fiction, meaning only 7 were nonfiction, almost an exact inversion of ten years ago."
They suggest some explanation:
fiction/nonfiction reading habits are extremely gendered, with 80% of women making up the fiction market, and nonfiction being a more 50-50 proposition.
and
Proportionally we are gaining many more women as readers and losing our few men.
EDIT: But they add: "The collapse of nonfiction is less a “male reading crisis” than a massive breakdown in the social fabric of trusting education, expertise, or any authority other than follower count and force of personality."