Well...I think it depends on the user...except in the case of netbooks (which I consider to be the equivalent of throw-away Internet-access vehicles).
Quite often...there is nothing wrong with a computer that a change of user would not cure...the machine is capable of performing for an extended period of time in the hands of an owner who does the things that should be done...to take care of a system.
I agree that systems are not replaced because they are no longer functional...but that train that develops technology will make them "obsolete" as opposed to "non-functional". When the user desires a more current system (for whatever reason), the system is replaced (in an era of system prices that have been on a downward spiral for many years now). It's given that a certain number of systems are replaced due to mechanical failure (like anything else man-mande)...but, IMO, the dominant reason for replacement has little to do with funtionality.
I think of the analogy we use for autos...3-4 year useful life (by the financial records) but capable of being used far longer by those of us not interested in purchasing new cars when the auto makers tell us to do so.
Fortunately, computers are much cheaper and the technology really evolves. Users would be better served by replacing computer systems and O/Ses more frequently than we do our autos