Windows is NOT the main OS because of any features, any user friendliness, nothing that has anything to do with the OS itself. Its the dominant because it used economical brute force to shove it down the throats of nearly everybody.
Honestly I just dont see this argument of some magical exodus of Windows users to Ubuntu happening within the next decade at the very least. There is no indication of it, in fact Microsoft has only increased and perfectioned their bully tactics and managed to force most people to Vista even with strong disapproval all around. They managed to force DRM into everybody, they are even threating the PC platform by attempting to turn it into a useless DRM happy appliance and nothing more. You really think a fugly brown Linux distro with Nelson Mandela and without painfully needed things like mp3 codecs not included by default will even threaten even 1% of Microsoft's market share?
The people who will come to Ubuntu will be ready to not be complete jackasses. If they were they'd probably run back to windows in 5 minutes after not finding "my computer" on this strange brown thing we use.