I'm not so sure of that. Their activation is a service that requires infrastructure at Microsoft. Microsoft can not reasonably be expected to maintain such infrastructure forever.
BTW, I actually stumbled across a site with details about the OEM's hardware activation just the other day. I never encountered this before because I always build my systems from generic parts. It's totally unfair.
Certain pre-activated makes of PCs don't use activation through Microsoft. The Dell Dimension will be able to run XP forever. The installer and Winlogon.exe check to see if the machine is prelicensed and skips activation. In fact, this is how some unscrupulous types have gotten around activation. They trick Windows into thinking it is a prelicensed machine, thus skipping activation altogether. Besides the obvious problem of legality with this approach, it possibly introduces another problem - driver compatibility. The correct driver might not install, and an incorrect driver that might install (assuming it will even attempt to install) may BSOD the system.
I've reinstalled XP on my neighbor's Dell so many times, and it has never asked for a product key nor ever said anything about being activated. However, the difference is in the machine, not the Windows files. If you try installing Dell's version of Windows on another machine, it will ask for your key and tell you that you must activate. The only downside is that you won't be able to run it on nicer hardware.
As for upgrades and stuff, well, if you already have the patches copied somewhere, then you can use them for as long as you want too. Their WGA only applies to when you download them. Very few things check for WGA status when you install them, and if Windows is hard-coded to already be activated on certain machines, even that won't be an issue.
The name brand stuff from manufacturers who already bought the license rights from Microsoft and comes with Windows have an advantage. Windows checks their BIOS information and does not require activation on them. Removing activation support won't affect these particular machines, and if you already have all the patches (or slip-streamed your own version of Windows which includes ALL the latest patches), then you are set. I slip-streamed a copy, not because of this, but because of trying to do a repair install of IE after upgrading the service pack. They would ask you to insert a CD which does not exist and call your original CD the wrong one. So the only way to get that to work would be to take the SP3 upgrade and the SP2 CD and merge them into a new CD image.
Still, I'd like to see us create an OS which will run on somewhat older hardware and not have activation nor artificial hardware limits based on which flavor you purchase (home, corporate, enterprise, etc). That is where we come in.