Indeed it isn't! But at the same time, neither is it Apple's and Google's problem that they are providing what end users want in mobile. Yet Microsoft fans act like the duopoly in mobile is a huge problem, citing lack of competition. Well, at least Apple and Google are competing with each other!
Microsoft doesn't have much of any competition in desktop, at least not yet. And it's not difficult to argue that the (lack of) quality control in Windows 10 is demonstrating the problem with that. Yet Microsoft fans have no problem with this.
My point is that mentioning the problem with a duopoly in mobile but not seeing an issue with a monopoly in desktop indicates that Microsoft fans don't actually have a problem with the duopoly in mobile. If they actually did, they would have a bigger problem with the monopoly in desktop. They don't. They simply aren't pleased that Microsoft wasn't competent in mobile, and failed. They would have no problem with a duopoly, or even a monopoly, in mobile if Microsoft's mobile OS was one of the options.