Android makes it possible to actually replace the launcher. Windows Mobile didn't do that, and that is where they failed.
I'm calling the shenanigans on this one. There were (are...) plenty of launchers for Windows Mobile.
If you knew enough about Windows Mobile to know whether you had a valid point, you'd know that you don't. Explorer is still lurking in the OS, waiting to pop up and make you fuck around with a start menu when your replacement shell shits itself. This sort of thing used to happen in Android, too... way back in 1.6.