Very good question. Let me try to explain, so perhaps this will help.
Most people replace their Windows machines, not because they cease to boot windows anymore, but because they are 'slow'. They seem to do everything slower, boot, shutdown, open applications - surf the internet ... what was once 'spunky' and 'snappy' now seems to be running in slow motion. So, the user decides to upgrade the system - because, well, frankly because we've been conditioned to do this.
Remember when the Pentium first came out? Then the 33/66 "Turbo" button? Then the PII and everything seemed to run 2x faster. Every 3-4 years meant almost a 100% improvement in processing power. Frankly, from the processing point of view, there really hasn't been an overwhelming reason for the average (non-gamer) user to upgrade their PC in the past 4-5 years. A Core2Duo chipset is more than adequate for most of what your average consumer will need - but, we've become to conditioned to change that it's our knee-jerk reaction to everything. "It's 4 yrs old, it's time to junk it".
And, to be fair, some of the newer OS on the market (I'm talking about Vista and Win7) have higher hardware demands, yet the overall performance in most benchmarks actually went DOWN over the same hardware running WinXP. So, there is literally a 'gun at your head' forcing the upgrade.
If you bought a Mac that runs the x86 (Intel chipsets) you can still run the older Mac OSX, as well as upgrade to the newer OSX Lion or even Mountain Lion. It's still running a Darwin Unix kernal; and this is perhaps the most stable and reliable OS known to man (Speaking of UNIX). I mean, let's compare your typical Intel laptop chipset; how can HP/Dell/Levino offer a motherboard that runs for 45 minutes to 2 hours on a charge, running Win7 Pro; yet the same chipset in a smaller and lighter laptop runs OSX Lion for 10+ hours? How is that even possible? I would argue that it's an efficiency in the OS that isn't simply not there in Win7, Vista, XP and likley not even in Win8 (although time will tell).
Once XP hit SP3, the RAM requirements went up - you could get by with 500MB when XP came out - but SP3 was maddening to use until you got above 1GB - 2 preferably. I saw a lot of this complaint - adding RAM became a routine fix for a while - until new machines started coming with enough RAM.
Seems like the processing power war topped out in the P4 days. Instead of one core running blazing 3-4ghz range, they seem to have settled on multiple cores running in the 1-2ghz range. Remember, memory speeds were always the bottle neck - now we have 1+ghz RAM.