1974 - TSR publishes (original) Dungeons and Dragons
1977 - Not a new edition, but the Dungeons and Dragons Basic Set was released.
1978 - Advanced Dungeons and Dragons is released four years after the original. This was necessary because the original wasn't written in a very user-friendly format. Materials are published during this time for both "Basic" D&D and "AD&D."
1989 - Advanced Dungeons and Dragons second edition is published 11 years after AD&D. The rules underwent significant revision and controversial content was dropped.
1995 - The AD&D2e rulebooks were revised (slightly) and reformatted but TSR was careful to avoid an misunderstandings - one of the first pages has an article that declares, "This is not third edition!"
2000 - Once again, 11 years after the last edition, Wizards of the Coast releases Dungeons and Dragons Third Edition. WotC took three years to develop this product. The rules underwent major revisions, most significantly moving combat onto a grid.
2003 - A minor revision to the third edition was released, called 3.5. This greatly irritated fans who had to repurchase the same books they had just purchased three years earlier.
2008 - Dungeons and Dragons Fourth Edition is released eight years after third edition and only five years after 3.5.
2009 - Paizo releases Pathfinder for D&D fans who don't want to move on to 4th edition.
2010 - Dungeons and Dragons 4th Edition Essentials is released, this was billed as 4th edition for beginners. Character creation was streamlined and the text redesigned.
2012 - They've been working on the rules for 5th edition for over a year now. We'll see 5.0 before fourth edition is old enough to go to Kindergarten.
So D&D has been around 38 years and we have five editions and at least two or three rules tweaks in between just for good measure. Shouldn't the core rules system have a longer shelf life than five years? I remember the good old days of second edition - TSR put out so much product for that game and it all seemed to be absolutely essential in some way. And it was GOOD too.
Ultimately, Wizard's impulse to cash in by releasing a new system may prove to be its undoing. Fourth edition was ill-conceived so now they have to release another edition to undo the mess they made. Ultimately, D&D becomes its own competition. I'm sure you could find gaming groups that still play every edition of D&D that never moved on to subsequent editions. If it ain't broke, don't fix it, right?
How about another option - instead of releasing another edition, apologize for 4th and simply make all of the books for all of the editions available as cheap PDF downloads and release new content for every edition! That way Wizards could profit from the legacy of D&D and it would make all of the fans happy!