GP talked about the choices of device manufacturers. If device manufacturers consider the sales potential for a new laptop model, are they going to ignore the competition by tablets? How about convertibles or tablets with a keyboard?
Consumers consider them somewhat interchangeable ("I bought a Surface to replace my old laptop"), so they IMO can't be excluded. And if you don't exclude them, then the share of windows laptops is now so low that adding linux support is a noticeable increase in the sales potential of that hardware.
Device manufacturers are quite cynical IMO. Would you personally take on extra testing burden to increase the sales potential for a new model by 0.25%? 0.5%? 1%? 2%? 4%? 8%? I postulate that the device manufacturers wouldn't bother for the numbers at the low end, but if they don't do it now, then they have a better reason than "windows doesn't require it" (quoting from GP).
I'd argue that the deficiencies in Windows as a laptop OS has led to some people choosing iPads and the like when they would've chosen a Windows laptop.
Obviously, a tablet and a laptop have divergent usecases, but there's some overlap. And for those consumers that exist in the overlap, I think many (maybe most?) would rather choose an iPad.
Microsoft kind of saw the writing on the walls with this and that's how we got the whole Windows 8 fiasco.
We are talking about laptops. Not tablets.
Windows has been losing market share to Macbook-s at about one percent each year, so good luck reaching your 20% in your lifetime.