I have a Mac that's been running OS X since it was in Beta and is now at 10.4 (the hardware won't go further unless I play games that I don't want to). Ditto on my 2001 iBook, started with 10.0, now at 10.4 (use it around the house as a netbook when I want to).
They've been through a LOT of upgrades.
And yes, Apple pushes out upgrades on a regular basis.
The degradation in Windows performance is due to:
1. Registry degradation over time
2. File system (directory) degradation over time(*)
3. Bloat over time
All of which are user controlled.
Most people think chkdsk doesn't work, with proof by running it multiple times still having problems. Most people also think that re-installing software to fix problems is a good idea. WRONG.
I'm the Director of R&D for a company puts out industrial-use software.
Our machines - Windows, Mac, Linux, other unices - are all under configuration control. We know what goes on them and when and why. All are updated regularly.
One of my testbed PCs is 7 years old, has been running XP since SP2 came out in 2004 - and it runs like a champ, just as good as Day One.
Mac and Linux and *nix have a filesystem advantage over Windows. You rarely run fsck (unix for chkdsk) on your own, yes, you can force it, and yes, it typically runs at boot - and rarely finds anything wrong, but it when it does, it virtually never results in serious loss.
Windows is more primitive by comparison and people don't run chkdsk often enough - as in before and after any installs and on a regular basis - and Windows doesn't particularly help with that, except on hard failure and even then, I've watched people skip it on boot complaining that it takes too long. And contrary to popular belief, you don't defrag a Windows system - that can usually lead to worse performance, in my personal experience (and as explained by a number of disk drive manufacturers - the days of simple HD storage schemes are LONG GONE).
And then, things get confused, the disk starts thrashing (because now the OS is thrashing) and pretty soon you get the famous disk crash.
Did I mention that I've been running two Maxtor 60 GB drives from 2001 on that old Mac? You know - the ones with the super-low reliability record in the Windows world?
It's all about management.
I feel that Windows is harder to use because without understanding and proper training, yes, your PC will degrade. Most people don't know that that's not inevitable. Microsoft has made pretty good operating systems since Win2k - allowing for the false starts and bad initial releases they're known for, so you always wait until service pack reports come in before upgrading the OS. (Win7 by the way, is Vista re-branded, so it was pretty good from day one in our shop.)
I feel that Mac OS X and Linux are easier to use because once you get used to them, they just keep working - the superior unix underpinnings see to that.
Then again, these are just my opinions and I could be wrong.