I don't know where you get the idea that O'Reilly books have a 6-month shelf life. That actually describes the vast majority of publishing, but O'Reilly less than most. A few representative titles that are still going strong (selling tens of thousands of copies a year) after ten to fifteen years:
Unix in a Nutshell - first published in 1984
Learning the Unix Operating System - 1985
Learning the Vi Editor - 1986
Sed & Awk - 1990
Programming Perl - 1991
Essential System Administration - 1991
Sendmail - 1993
DNS and Bind - 1993
Java in a Nutshell - 1995
And these aren't just a few outliers, though they are some of the best known. We have hundreds of books that sell year in and year out. And it's precisely because we engage with our community of users and keep awareness of our backlist that this is true. It's the frontlist publishers, who rely on marketing and store placement, who are living with short shelf life.
Unfortunately, what you say "is not the standard publishing model" IS the standard publishing model, and that's the problem.