Chromebooks, iPads, Android devices... these are mobile devices and are a completely different market. There is a reason many people have a phone, tablet, laptop, and a desktop. Since laptops and desktops differ only in hardware flexibility and performance and can almost do the same things these days they are becoming comparable. Mobile devices on the other hand do not run with full desktop capabilities and interface. Most people have both a desktop device (laptop/desktop) and a mobile device (tablet/phone).
As for Linux distributions and Macs, certainly, they are viable in many ways but MacOS only has 4.9% of the market and linux 2%. Ultimately yes, it does come down to market share. It also comes down to the fact that for almost the entire market changing operating system after purchase is not a viable option for their technical skill level and learning a new operating system is not a small undertaking.
Imagine a world in which Oxford has control of the English language and makes sweeping changes, say causing every word of English to be heard by the NSA. In this world switching would mean losing all knowledge of English. Your argument is akin to suggesting they no longer have control of the market because people have morse code, french, and german as viable choices. Morse code is a different sort of communication mechanism. One could switch to French (4% of speakers) or German (2%) but then you couldn't communicate with the 90% that are using English and if publishers don't choose to print their books in your language which only 2-4% of people understand you can't read them, the same for movies, music, software interfaces, road signs, and of course you are stuck with this choice if your place of employment opts for any of those things that are English only.
The simple reality is that so long as Microsoft has a monopoly, people making a product or service that integrates with a computer or software can target that platform and reach 90+% of the market. When it costs just as much to target each other platform many can and do choose not to target the other 6%. You can make something web-based and possibly hit 100% and the mobile market as well but that requires a browser to access and Microsoft is shoving their browser down everyone's throat.
If they succeed suddenly they have the power they used to have with IE, the power to introduce new and incompatible features and make all other browsers look inferior simply because they have different features than what people are used to. Suddenly web content starts to have the same problem it used to have when MS had a browser monopoly a bunch of MS browser only targeted content. It would roll back advancements in open technology to the terrible state they were in during the 90's. In many workplaces people are still locked in to inferior IE only content due to mentalities and carry over solutions from those dark times. Nobody wants that... except Microsoft.