Windows 7 is not an anomaly, the OS sales numbers should be on an increasing trend. Windows 8 is the anomaly that is comparable to the failure of Vista. I am pretty sure that they could have achieved a sales better than Windows 7, if the product was really compelling.

Unlike in the past, the OS upgrade cycle has shortened considerably thanks to mobile devices. People don't really mind upgrading their OS even though Windows users might be more reluctant than others. But I am sure that the Windows 8 user interface changes are the real reason why most people would not even want to upgrade.