I hear this line of reasoning all the time, and boy does it piss me off.
Because I should spend all the money I get from my job on Apple hardware, and not little things like saving, the kid's college fund, etc.
Yes, computers have a lifespan that in my opinion is about 4 years. So, folks try to plan purchases into their budget around that schedule. If a hard drive dies at year 3, most people are more likely to replace the hard drive rather than buy a new computer a full year early, especially when money is tight.
Over 25% of Apple's Mac OS X install base is still running Leopard or prior. Apple should really consider working better with those folks.