Actually, he might have a point..
Generally I'd agree, if the system is functionally complete, stable and has no security issues. Unfortunately over time this starts to become an issue.
I've just de-commissioned one NT 4 box that's at least five years old and probably much older (yes, it shouldn't have been used in the first place if it was installed after 2000), and have another one to go.
As soon as a security problem or unexpected functionality issue occurs, you are utterly stuffed. There are no longer any patches available and the wealth of knowledge about the product has suffered from bit rot and memory loss - old web sites are taken down and people move on to new things. The end result is an often fraught replacement process.
The situation isn't necessarily much better with Free software. Updating to several releases beyond your current version can be tricky, hardware requirements slowly increase, older hardware is dropped and/or suffers from code rot - the code is still theoretically there, but no-one bothers testing it any more and it starts to fail. The advantage is that you can update frequently, usually for no cost..
Of course, that might not be so much of an issue with home systems, but if the Steam hardware survey from 1996 is examined, a sensible proportion of systems are incapable of running Windows 7 well (yes, I know it works well in low memory situations, but realistically you're looking at 1GB+ RAM and >100GB storage to run modern software - otherwise why switch?).
Obviously Steam is a snapshot of established machines, which may already be a bit old, but as it's a gaming survey I think it could be argued that any machine older than five years old isn't worth upgrading/needs to be skipped.