Reasonably successful? I dare you to name one single Nokia-phone made the last half decade which you would consider relevant.
The truth is that platform-wise and software-wise, the bits you interact with, they've been stagnant since around 2000 and no amount of megapixels or Carl Zeiss optics will be able to compensate for that. They seriously need to throw their old stuff away and start going on something new. Like Microsoft did with Windows Mobile prior to Windows Phone 7.
I'm honestly surprised they didn't do it years ago. Now they've finally gotten a CEO which actually understands this and is willing to act on it. About time! While Microsoft is a risky bet, this move may actually save Nokia from complete annihilation and is certainly better than doing nothing at all.
And so far, Windows Phone 7 looks pretty good. Care to elaborate on how it is a failed platform already? Not disputing it if you have numbers, just that so far I've actually seen more enthusiasm for it than I expected to see.