Sales aren't generally weak, just sales of Windows products. Android smartphones, Android tablets, and Apple tablets are selling very strongly. People can make all the excuses they want, I say people are interested in tablets just not Windows 8 tablets.
As for desktop PCs... Sales have been declining for a while but I've yet to meet a single member of joe public that's brought a Windows 8 computer or laptop and not absolutely hated it. The Microsoft apologists can make all the excuses they want, the evidence I see is that the public just doesn't like the direction Microsoft are taking with Windows.
However, you left out sales of Macs. Macs alone (portable and otherwise) buck the picture you are trying to paint - in fact, sales of Macs are suffering from a larger decline in terms of percentages than sales of PCs running Windows.
Then there is the decline in sales of iPads post-launch of RT - that's real. I'm not saying that all of the decline is due to RT; however, almost certainly some of it IS due to RT - RT sales came out of someone's hide.
And nobody (not even those of us that like and use Windows 8 without difficulty) is saying that there wouldn't be consternation over the Start menu being gone - most of us, in fact, have stipulated it. However, as has been the case with MOST objections to major change - in anything - the reasons for the consternation are subjective - NOT objective.
And that is, in fact, your REAL issue with Windows 8 - you can't get past that major change and have no interest in actually trying.