Are you saying that because large numbers of typical users have not yet switched to desktop Linux, we can conclude that desktop Linux is not a reasonable option they can switch to?
No, the fact that Linux is not currently a practical option for most users isn't proof that it can't be or never will be.
I am saying the lack of desktop adoption is indicative of the difficulties of doing so. There is a level-of-effort barrier and technical-knowledge barrier to it. 20 years of progress have lowered those barriers a lot, but even if something like Ubuntu will often be fully functional with a standard install, most users never have to install an OS. They can't walk into Best Buy and come out with a computer that runs desktop Linux.
I think the success of Chrome Books show that people would be receptive to alternative operating systems, but we don't have a retail or post-purchase support environment in place to facilitate it, and I don't see that coming on the horizon.
