I work in a pretty busy independent bookshop. SP was selling very well when the film launched. Last week, following the media revelations, my managers decided to leave a pile of SP in a prominent place by the till thinking some new sales might be being generated and I deliberately kept an eye on it. A few customers read the blurb on the back. One, who was buying a different book, had a brief chat with me about what they were reading in the papers, but no copies of SP, or Winn's other books, actually sold.
Creative writing about nature or nature-based travel has increased massively over the last few years. The UK's Wainwright Prize is a barometer to that. From 2014-2019 (I think), there was just one award for nature writing. SP was nominated in 2018. The genre has expanded so much that by this year there are now 6 awards, each with separate sets of judges: three for adult's books and three for children's. If I've counted correctly, this year's longlist nominations add up to 35 adult and 34 children's books in total. New and better writers than Walker are emerging all the time. There are now UK Master's degrees (and many other courses) which specialise in nature and travel writing, adding in a new stream of motivated, potential new authors as well.
Regardless of the truth of what she has already written and the outcome of the current publicity, the market for Walker/Winn's work will surely sink. It's already being overtaken.
On a different note, I've seen nothing yet about not-for-sale, advance proof copies of On Winter Hill being available anywhere. Publishers send lots of books out in that format (often with blank, or draft covers) to named reviewers for endorsements to be printed on the covers, and to bookshops and online or print reviewers just to drum up the pre-sale publicity. It could be that PRH had an early-enough heads-up about the initial Observer article that they managed to hold off letting any proof copies leave their control. They will have been aiming for a major pre-Xmas launch with their original October publication date.