This article from 2006 estimates between 50-100 authors of SF&F earn enough from writing to support themselves.
I agree with those who say it's always been tough to support yourself purely off writing. Certainly off writing fiction and selling books.
Writing, like acting and making music, is one of those things that there are more people wanting to do than there's an audience for. So from a crude economic point of view over-supply drives down prices, fragments the market and makes it very hard to make a lot of money. With ebooks and self-publishing this is exacerbated. There are those who simply jumped on the band-wagon and when they realised that they will only ever make a few sales here and there will leave. But equally there are those who are just happy to be being read and for whom the extra money is a bonus.