In practice, we hang on to old computers until the threat of hardware failure induces us to upgrade to new hardware. And then we get the flavour of Windows of the day that comes with it. We are usually not motivated by the need for higher performance and never motivated by the need for new features (with the exception of WPA2 support on a couple of old netbooks).
The only OS's that we have actively avoided (so far) were Vista and ME. We actually bought 7 used PCs with XP Pro licenses during the Vista period, largely because we didn't want to replace all of our peripherals (printers, scanners, etc.) at the same time. Half of our machines are still running XP. The other half are Win7.