On the matter of novel sales, I don't know whether they as a whole are the bulk of their D&D income or not. It's more likely that Drizzt alone gets WotC more than the rest of FR. I say this because they drastically cut down the number of yearly novel releases to 3 books, and 2 of them are always going to be Drizzt ones. Furthermore they reduced the number of authors working for them to 3-4 persons: Ed Greenwood, RAS, Erin Evans and perhaps Troy Denning. Really, the situation of FR fiction looks as grim as that of the setting material.