Fixing some buffer-overruns doesn't require the whole Windows crew. A multi-billion dollar conglomerate could surely create a skeleton crew for this task. Especially since this service wouldn't be free. Or just keep the ones who are currently assigned to fixing Windows XP still at it.
Think about it, it does not only affects Microsoft. As application developer (that aren't inhouse), need to support 5 differents "Windows" at the same time adds tremendous burden to QA (if your company has QA at all, though). I personally hope WinXP die soon so that we can skip Vista too and directly declare our minimum requirement to be Win7.
Let's not forget Microsoft is also releasing their own software package known as MS Office as well as many other much smaller applications. They have large incentive to wish WinXP fade out soon too.