Re: "a new version of OS used to trigger purchases of new PCs."
Up to and including Windows Vista each successive iteration of Redmond's OS virtually mandated a new pc (or a significant hardware upgrade) in order for it to run with anything like decent performance. Microsoft's minimum recommended specs were always a bad joke given that a on a machine that had those specs the iteration concerned could just about get out of bed. It was always the case that you needed substantially higher specs in order to get reasonably acceptable performance. This changed with Win 7. For the first time if you had a box which had ok specs for Vista then Win 7 would run with an acceptable level of performance. If you then add to that the transformation of the private retail market in terms of the rise of mobile devices meaning that you have phones, tablets and PCs all competing for the punter's hard earned one has then a situation which IMO cannot be fully explained in terms of disatisfaction with this or that iteration of Windows. I would agree that Win 8 was very clearly not any kind of hit with private retail customers but that is not remotely an adequate explanation for where the pc market has been going in recent years. Indeed if we look at the tablet market we can see that it also began to stall about three or more years ago and the mobile phone market began to show the same signs a year or so ago. The Windows OS can scarcely have anything to do with those phenomena! I think that the processes at work hear are market saturation, maturation and three different device classes trying to attract the punter's spons in a context where people do far more on their phones than they ever did before thus making the PC a lower priority than it once was. If we add to that the fact that modern tablets and smart phones continue to have perfectly adequate performance for several years it becomes clear that the whole personal tech retail market is becoming a very difficult place in which to do business.