One OS, one device (or a small handful of very similar devices), and one app store is simpler, but not necessary. What didn't work was the crazy panoply of design options of the windows mobile world, where screens could vary from a large HTC device to a tiny clamshell screen and where you could have a number pad on one phone, a full hw keyboard on another, and a touch screen on another. That degree of diversity is difficult to make high quality apps for. However, android is a bit better, there is less diversity and especially fewer giant gaps in capabilities. The amount of additional effort it takes to write an app that works well on a Droid 2 as well as a Galaxy S is insignificant next to the comparable effort for targeting both an HTC 8525 and a tiny clamshell phone with no kb, no touchscreen, and a 120px resolution screen. The desktop PC world manages this challenge just fine.

And ultimately catering to users' particular individual needs is likely to be a better market strategy in the long term compared to the "one true device for everyone!" ideal of Apple.