The article admits there are gaps in the statistics due to the metoedology of basing it in the OS of systems accessing the internet but what about those that do not use the web at all.
From those that sit on a desk in a small non internet based business used only for WP to those companies who restrict which employees have access to the internet as a timewaster there is going to be a considerable count worldwide. As these are probably going to be predominantly older OS's I think that XP has a long way to go before it loses its crown especially as the differential is so small.
Add to that all those systems using one of the flavours of Windows as Embedded controlling various dedicated machines and systems which are not in the market of regular upgrades because they are designed to be a particular job.
Win8 is a major change to the OS is used day to day even if you think that change is acceptable. I am still getting used to it but such annoying things keep coming up from how to get the Run box to shutting down. The music tile opening XBox Live, why? I have 18000 tracks so why is the 1st thing I am going to do is buy more? Most of the "apps" I use promptly drop back to the desktop so I might as well stay there