Why can I say you're wrong? Because people have been saying that for 20 years, it hasn't happened, it won't happen, it isn't even remotely close to happening.
I can remember much the same being said about Internet Explorer, which went from well over 90% usage share to more like 20% over the last 10-15 years (with much of the decline happening before mobile became an important factor).
An entrenched monopoly can be difficult to dislodge, but that doesn't mean it will last forever. Microsoft has also lost a lot of ground that would have protected Windows had it held onto them - control of the web browser and wordprocessor being the two main ones.
(Imagine if every website used ActiveX - that would be a problem for competitors. There are plenty of market niches were similar problems still exist, but for mainstream users I don't see any insurmountable barriers to migration now.)
Now it may very well be that what replaces the Windows desktop isn't called Linux. It might not even be Linux-based, or run on what we would currently recognise as a desktop PC. (The most effective challengers so far have been Android and IOS, which satisfy two and three of these conditions respectively.) Microsoft could also stay there longer by upping their game. Nothing lasts forever, though.