Well that's the problem!
In the past, a PC gamer would replace their main rig every year to 18 months, and this would drive quite a bit of sales. In fact, ordinary PC users would change their computer every 2 to 3 years because the new ones were much better, and new software was more capable (and a lot more bloated) and wouldn't run well on a 2 year old machine. This started changing in the early 2000s for non-gaming PCs (my non-gaming development box I built in 2002 lasted 7 years - basically until components started to fail). For gamers this started changing towards 2010 - now there's little advantage in changing your gaming rig more than once every 3 or 4 years.
The result - while PC usage is probably still growing a little, PC *buying* is declining rapidly because a machine from 2010 is still good enough even for gamers, and a machine from 2005 is good enough for typical email/browse the web stuff. My main gaming rig now is a decent spec *laptop* with nvidia graphics and an i7, and not a hideously overweight one either like gaming laptops of 5 or 6 years ago. Since hardly anyone buys Windows retail, falling PC sales means falling Windows sales. A Windows license for a normal PC is lasting 6 years or more now as people only replace when components actually fail beyond economic repair, and most every day users are no more likely to buy a Windows upgrade any more than they will switch to Linux. A Windows license on a gaming PC is lasting at least 3 years now, possibly more - when in the past, Microsoft could rely on gamers buying a new Windows license every year to 18 months and non-gamers every 2 to 3 years.