But they will. The 4e core sold well. The ultimate "problem" for its longevity (not for me, I love 4e) was that it was too different, too much of a departure. As long as "6th edition" is still compatible with 5th, they can perpetuate sales and freshen up the brand without alienating fans who feel like they have to keep up without being obsoleted. "6th edition" doesn't have to even sell exceptionally well; just as well as your average splat (enough to make money and increasingly grow the fan base). And it doesn't have to be every one or two years; four or five is probably better for D&D. The problem is that WotC sells 3.5 and 4e Essentials as one time corrections in the middle of an editions life cycle rather than natural evolutions overtime. Going forward, I'd like to see all future "editions" to be fully compatible with prior iterations. An ever growing catalogue of rules modules will supplement the evolving base game. If WotC makes the revised rules available for free as they come out, we should all be able to enjoy the game in harmony, and unlike editions prior to 5th we will all still be playing the same base game.
Arguably, this should have been the model from the very beginning. Instead, WotC introduced (somewhat successfully) Magic 3.0!... Magic 3.5!... (and somewhat less successfully) Magic 4.0!.... Magic 4.5 Essentials!....
Magic: the Gathering would likely not have existed this long if it had been presented to players like this, where each new set was so different from prior ones that even the card backs were different, so they couldn't be played in the same decks.