Your view contradicts publicly available information. Again, I encourage you to look at the information coming directly from Amazon and from the store woners themselves. I am not guessing. I am not even introducing the view - I'm just repeating what the people that are directly and financially dependent upon being right about these things are saying.
They are selling - but at a tiny fraction of what a new book sells.
Put yourself in the shoes of WotC. You can either: a.) Continue to sell a book that is over 2500 down on the best seller list, or introduce a new book that refreshes that material and have it sell in the top 10 (before it, too eventually dies off).
Next time you're in your FLGS, look at the RPG shelves. Now, ask yourself what will change when they're trying to shelve the new version of D&D, the new competitors to D&D, and everything currently on the shelf. What is going to give first?