not that I believe this particualr case, but even if past trends of D&D would be once every 10ish years, the trends of virtually every other game sytem would probably be every 3-5 years. It isn't that inconceivable to me that wizards/hasbro might decide to alter their marketing strategies to conform with the industries standards of a new edition every 5 years or so.
D&D is usally more suplement rich than other game systems so a new system should come out elss frequently than the norm though I'd think.