>If that's how a lot of technical authors are chosen, that could explain a lot about the quality of a lot of books.

I agree, but I don't see how self-publishing makes that problem better and not worse.

>meaning that less books get written.

I have a hard time believing this, in the age of the internet. For one, there's more content produced than ever, and it's still accelerating. Second, do we need more "books"? Why not essays and papers and blogs? Not every idea needs to fill 256 pages; most are best served with a less lengthy medium.

But yes, writing a book is a form of art, like making an album or producing a painting. It's very hard to make a living doing so.