If you buy Windows and have trouble installing it (which, by the way, is far less likely in my experience than with most Linux installs -- although things are getting better on that front, too), there's a phone number I/my grandma can call for help. You'd be surprised by how big a difference that makes.
Yeah, I'm not suggesting that the Linux installation process is as friendly as Windows (though, as you said, things are getting better). But how many Windows users even know what an operating system is? How many of them realize that Windows isn't built-in to the computer? I think there's a mental obstacle here that would make installing Windows difficult for many (but not all) Windows users.
I mean how many users could take a PC with no OS and get Windows running on it.
Installation is an obstacle for linux adoption, but it's not (entirely) because the installation process is broken. It's because Linux doesn't have the OEM distribution model that Microsoft does.
I don't think this refutes anything you've said, I'm just pointing out that it is more than a software issue.
