This is actually where everyone who is trying to defend Windows 8 is wrong. You can look at computer sales trends vs usage rates across Windows platforms to see how a new OS is being adopted. Sales of PC's in general have gone down, but not nearly at the rate in which Windows 8 is underselling or underadopting Windows 7. The adoption rate is like 25% or 30% of what Windows 7's was during the same time period. PC sales have not dropped 70% or anywhere near that since Win 7 came out. So there are 2 options, neither bode well for Windows 8:
1. People are actually asking vendors for Machines that have Windows 7 installed instead of Windows 8, which means they really don't want Windows 8.
2. People are downgrading from Windows 8 back to Windows 7, which means they really don't want Windows 8.
Anyway you spin it, consumers simply don't want Windows 8.
Add to this that some vendors have now actively come out against Windows 8 due to lack of demand and complaints and articles like this one:
http://www.reuters.com/article/2013/04/10/us-pc-data-idUSBRE93914P20130410
And its pretty easy to see that the majority of MS's core users hate Windows 8 and want nothing to do with it. You can't ignore the mountain of circumstancial and data evidence just because you can't prove 100% correlation.