I have a much simpler explanation. Publishers and conference organizers like O'Reilly eventually saturate the market for Perl books and conferences (books especially, because used books start to cannibalize their sales), so they move on to promoting a different, new language, so that even their existing customers will have to buy the new books and pay hundreds/thousands of dollars to attend conferences for the new thing.
I am not sure the total count of how many books O'Reilly published last year, but they have not exactly slowed down in releasing new titles: http://shop.oreilly.com/
In SF, maybe. But most developers in the world live outside of SV (for example, there are on the order of 1M C# developers worldwide). They don't write blogs, or even read them.
Developer outside of SF (outside of USA, in fact) here. I read blogs, have written them on occasion. Have worked with many developers in many non-SV locales. None of them read programming books, most of them read blogs.
I have a much simpler explanation. Publishers and conference organizers like O'Reilly eventually saturate the market for Perl books and conferences (books especially, because used books start to cannibalize their sales), so they move on to promoting a different, new language, so that even their existing customers will have to buy the new books and pay hundreds/thousands of dollars to attend conferences for the new thing.