Whole industries have died because a free product has emerged that's "good enough" for 90+% of people. When was the last time anybody bought the electronic Encyclopedia Brittanica now we have Wikipedia?
Such a drastic change of OS basically disrupts your whole workflow while you get used to the new one. Everything from where to find things to keyboard shortcuts to UX paradigms.
Its fair to say that even the Win7 to Win10 switch is something people only do if you force them, but I could see the Win10->Linux change happen for a lot of people if they saw anything it had that they needed.
Not having to worry about AV and automatic updates, automatic download and in-OS-ads would be pretty huge already.
Yes, most powerusers on windows know where to turn all that off, but a lot of people, like my parents, are scared to fuck something up, so they dont touch any settings, especially not if windows tells them "this improves your experience, you sure you wanna turn it off? nudge".
A lot of people would not notice or care if you switched their machine to Linux - as long as it means they have to worry about less.
It would be great if Steam had non game applications.
When tax dollars are still being shuttled to the deep pockets of MS and Oracle, a more proactive stance is needed.