I don't think the whole Win10 thing is a major factor here. Most people outside IT don't really care what Windows it runs, beyond grumbling a bit when stuff changes.
I think the main reason for the slump is that, back when PC performance was doubling every 18 months, developers made use of the extra oomph either by adding more features or being more lazy, until you would be basically forced to upgrade, because your old machine would collapse under the weight of the new version of something.
Now that PC performance is pretty much stalled, any application that's targeting recent specs will usually also work respectably on fairly old iron. So what's the point of upgrading?
We may see a boost to sales once they get SSD costs down to the point where developers feel they can get away with building stuff that works poorly on HDDs. Then everyone will be forced to get an SSD, and many of those will just buy a new PC.
Biting the hand that feeds IT
