Because the already-done software (e.g. XP) has serious issues; namely, even if you have obtained it legally, you can't legally install and run it without the manufacturer's explicit consent (activation servers will go down some time after EOL - after that, you can either go illegal and crack your copy, or you can't use it at all). In other words, it's timebombed. (Note that I'm not passing judgement on this - just stating the existence of such condition.)

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).

Or Microsoft could issue a hotfix to remove the license check. Adobe has such unlocked version of CS2.

Which is something which might happen, or perhaps not, and is entirely under MS's control and discretion.

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.

It is easy for me to imagine them doing it, if they wait enough time, that most new hardware has no XP drivers anymore, and most programs/games depend on higher versions of windows APIs, the general public will not care about the then obsolete XP, just like they do not care about DOS, or Windows 3.11 and so on.

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.

Haven't we learned a fair amount since the days of XP-specific software? I've never seen an enterprise XP application that was "done." Only shipped. Don't we have better platforms now that make it easier to write responsive and stable software?

Why waste time rewriting Unix...