If most PCs today start shipping with a Linux distro then you will see how much of the Windows Market share comes down.. Why it does not happen because Microsoft has an absolute control over OEMs... That is how they grew in the first place and kicked all the competition out through bribing and predatory practices... Even OS/2 was not spared which partially had Microsoft code in it.
@safiuddinkhan I don't think it's that easy. True, they use whatever OS came with their device, but most take the OS into account when choosing what device to get. I can't imagine a person who wanted an iPhone, but then went to the store and got an Android device. If they decide to switch, they do it before going to the store. So if most laptops come with Linux they just might look for the ones that ship with Windows.