The book-scanning process in Rainbows End was an obvious parable, not an attempt at plausible futurism. Even in the book, alternatives to the destructive scanning process existed, and the choice of digitization technique was a business decision, not a technical one.
Furthermore, many of my favorite authors (Stephenson, Pynchon, Murakami) work full-time as writers, and seem pretty capable of doing so without starving for source material.
I may not have given it a fair reading, because (personal preference) I really didn't like the book; the cyber stuff seemed more like unoriginal shiny tech than a way to provide insightful commentary on what it is to be human - unlike Tine pack minds, for example.
I haven't done a survey of fulltime writers; and I'm sure exceptions exist as you've noted. Larry Niven knows many more writers than I do, and so I think his observation probably has some merit. But really, I'm just expressing my frustration with Vinge's output, and noting that it coincides precisely with his retirement. I miss his old stuff.