Since when does doing well preclude wanting to do better? You think successful companies rest on the laurels of "good enough"?
Not at all, but there's a difference between going forward and going back. 4E staked out a lot of bold new ground in areas like PC races (new PHB races were introduced for the first time since AD&D) and class mechanics (fighter dailies, wizard at-wills, and so forth).
If 4E were a big success, one would expect the game's continuing evolution to build on those changes. More exotic races, more fancy new classes and class powers. Instead, Essentials is retreating somewhat; dragonborn, eladrin, and tieflings have been banished from the new Red Box and the classes are reverting to something more like previous editions.
Obviously most of these changes reflect the sensibilities of Mike Mearls, an old-school gamer who blogs about his AD&D campaign. But why was Mearls tapped to take the helm? My guess is, because 4E is regarded as... not a failed experiment necessarily, but a partial success at best; too many old customers lost and not enough new ones gained. So they're pulling back and seeing if they can recapture some of the folks who left, and maybe tap into whatever magic drove the BD&D/AD&D craze back in the 1980s.
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