> Similarly, Windows could be losing users to smartphones in general

But this is desktop only. If someone stop using windows completely, it won't show a decrease in windows usage. This will basically only show when people switch from desktop OS.

You sure about that math? If there are 2 linux users and 8 windows users, then that's 20% linux, 80% windows. If one windows user quits using computers, then that's 2 linux users (out of 9 total) and 7 windows users (out of 9 total), or 22% linux and 77% windows users.