Exactly. First, I may not have to buy all the "re-hashed" books, but new players would. Second, its not uncommon for a new edition to "re-hash" older material. I am less likely to buy a new game where what changed is the rules and not the setting/background. Everyone in my group (in Germany at the time) stopped playing WFRP when 3E came out and when I got to my new group (in Maryland) no one was interested in 3E either. Everyone was comfortable and satisfied with 2E.
It can be profitable to re-launch 4E as a revised 2E instead of 3E revisited. First, as I said earlier, they don't have to worry about GW trying to keep their new nifty dice mechanic system. Second, there is a long history of great games in the Warhammer Universes that use that system of a modified variety of it. Third, they can (as some one else mentioned) focus on areas long left untended, such as The Moot, Elves, a real Border Princes book, Lustria and so forth. The Warhammer world provides a lot of background and possibilities. To think the sales of the game is limited to its "rules" is quite silly. Even when the fantasy battles miniature rules "change" they barely change, just enough to get the people to buy a new book that has pretty much the exact same fluff.
So yeah, you re-use the fluff, revise the stats, upgrade the system and target different sections of the setting and different styles of play in stand alone RPGs (it seems to be working for 40K). Hell I would buy the FFG 3E books if they had 2E stats for them.