Over the long run, mobile is king, and windows isn’t there. If Apple or Android figure out how to take advantage of their situation on this, they could overtake windows in 40-50 years.
Is that also your overall estimate on how long the mobile paradigm will overtake the desktop one? I would guesstimate that to a decade, or a decade and a half at most, with the degeneration of written language following closely.
I say they did, in that they have to treat other OSes as "real" - if they had dominated as they did in the 90s and early 2000s they would have just made Office for Windows and left everyone else on some Web version.