I don't see how a decline in sales has anything to do with windows8. This correlation is a mere assumption and has yet to be proven and should therefore be stated as such. I am by no means a fan of windows8, but I am also no fan of such biased claims.
In my opinion there's simply no need to upgrade to a newer PC these days. I am working in a industry where it's important to get as much computing power as possbile for a reasonable price. We usually upgrade all our machines in 2 year cycle. But these days there simply is no need to upgrade. My 3 year old machines run just fine. So in my opinion moors law coming to an end is one of the reasons for less pc sales.