They really could make money on an expansion to add some of the things from the first edition expansions.
They need to show how releasing an expansion would grow the business as a whole. Selling a product only to a narrow segment of the market who already bought the game is not going to make as much money for them as a whole new game, which could potentially act as a gateway product for new gamers to buy their core products (WHFB & 40K) and in the end, that's the GW business model. I'm no fan of it, as I don't care for either of those games, but unfortunately that's just the way of things. They aren't the only publicly listed company to have a myopic view of their industry.