That's one way to look at it. Another way to look at it is: Why do we not *update* these books? This gets back into the question of copyright of course.
Because the human brain has the capacity to frame specific pieces of work in their time periods, understand the discrepancies between past and present and work around it.
I would also like to offer an additional counterpoint: Leaving work intact will, not only preserve the work "as is", but also contribute to future historians the true sociological impression that is aptly accurate for their time period.
Because then we lose any sense of historical context.
Old books at their essence lets us talk to people from the past. We won't always like what they have to say, but they will speak about themselves and highlight how we've changed. If we can't understand where we came from, then we can't hope to understand how we got where we are.
What? Tons of books get new editions. It happens constantly. For example, Strunk & White [0] has been updated in 1918, 1959, 1999, and 2009. Authors release different versions of their own books. I just wrote a review of a book [1] that had a 2nd edition released in 2022 after the first edition was released in 2014. J.R.R. Tolkein retconned The Hobbit after publication of Lord of the Rings. [2] (I realize this isn't really the same thing but I wanted to give an example in fiction)
Anyway, point is, tons of books are changed after publication. I don't see why The Mythical Man Month shouldn't be; the technology is out of date and the gender issues are out of date.
I don't think there's much evidence to support your viewpoint.
Most books don't age well.
I tried to read Baum's "The Woggle-Bug Book" a few years ago, and found its humor, rooted in old ethnic stereotypes, totally un-enjoyable. It did not age well.
While the Hardy Boy books I enjoyed as a kid wouldn't have been so wildly popular had they kept the original text. ("From 1959 to 1973, the first 38 books were extensively revised, largely to remove depictions of racial stereotypes" says Wikipedia.)
One of the most popular Agatha Christie novels is "And Then There Were None". That was "re-published to align with the political zeitgeist" so much they even changed the title and the name of the island. Most people have read the re-aligned version, not the original version.
And as I learned recently, "Charlie and the Chocolate Factory" was "re-published to align with the political zeitgeist" in the 1970s, due to complaints by the NAACP and others that the Oompa-Loompas were African Pygmies. The re-write made them white, and the movie made them orange. The re-aligned version has outsold the original version.
Also in the 1970s, Geisel ("Dr. Seuss") edited "And to Think That I Saw It on Mulberry Street" to change how it describes and portrays a Chinese man. That book continues to be sold.
Carnegie's "How to Win Friends and Influence People" was revised in the 1981 edition to reflect the new feminist zeitgeist. For examples, "insurance man" was replaced with "insurance agent", and "leadership gravitates to the man who can talk" changed to "leadership gravitates to the person who can talk". It also dropped the section on marriage. ("Never get married until you have kissed the Blarney Stone. Praising a woman before marriage is a matter of inclination. But praising one after you marry her is a matter of necessity — and personal safety.") The preface to the 1981 edition points out how "Carnegie himself was a tireless reviser of his own work during his lifetime" and would have made changes because he was "sensitive to the changing currents of present-day life".
While not "tons of books", the fact that I - with little knowledge of book publishing history - can point to several counter-examples, suggests that there are a lot more popular books out there which were updated to the modern zeitgeist.
[edit] With Economist link: https://www.economist.com/britain/2023/03/23/editing-roald-d...