IMNSHO, this is a very unlikely scenario - the (misguided) reaction of the general public would be something like "oh look, XP had its antipiracy protection disabled by MS, which means it's now free to use; who needs Win8 anyway?" I can't imagine MS essentially undermining one of its core cash cows (Windows) anytime in the near future, especially as the "meh, just get a new computer with new Windows" line has proven effective in most cases. Also, as I have yet to see this to happen to any deprecated and truly obsolete MS product (Altair BASIC? MS-DOS? Windows 3.11?), I don't see that happening to a product which is anything but disused.
I agree that this is an act of kindness, that can not be depended on and it is entirely under MS control when/if it will happen.
For a home user, this is a non-issue (as "meh, just get a new OS" is also an option, due to commodity hardware); for an organization, this is big trouble (as neither "so we'll use the OS illegally, whatever" or "so we'll throw away the equipment we now can't use" is an option).