Won't necessarily be "a day". Rather, it's a slow but accelerating erosion of what was once a near-total monopoly.
The ease and low cost of deploying Linux in VMs -- on desktop/laptops and in cloud networks -- has tons of people being exposed to and converting to Linux from Windows. Even the Mac has seen growth from Windows escapees, despite its historically higher acquisition cost (because Apple doesn't do low-end).
Windows won't die for a very long time, but only because it used to be the only game in town. That just isn't true any longer. Windows 10 isn't a bad OS, in fact I'm getting used to it and finding it's quite usable. But the high pressure tactics of the past year are taking a toll.