It is possible that 4e was a failure. It is also possible that 4e did just fine and made a tidy profit, but the writing is on the wall that it will not continue to be as much of a success in the future. These are not the same thing, but both are reasons to move into developing a new edition now. However, people tend to conflate the two, or forget the latter is an option.
So, part of it depends on your definition of "success". Is it a success if it earns you a boatload of money for 4 years? Or is is a success only if it does so for 8+ years? Who is making that determination?
My definition would involve the concept of "meets or exceeds expectations" - particularly for Hasbro (and I'm talking about success from the corporate POV) its Return on Investment. For Hasbro, success =/= "makes a profit". Success = "Makes more return than we could have made by investing the money in something else".
Even if 4e did brilliantly 2008-10, we know it was cancelled early. If the plan had always been to terminate 4e in 2011/12, then they would have had 5e ready to go in mid/late 2012, right? Not a 2-2.5 year gap. Or do you think everything is unfolding just as Hasbro predicted? They're Cylons with a Plan?