I can't see the point in not fixing IE for XP. The flaws were there long before Microsoft stopped releasing the (mostly useless) "security" updates for XP, and the browser should be seen by the company as an independent entity than the OS.
If the browser had been seen as a different entity all along, they would've stopped pushing updates years ago for that version. They're now at version 11 and most browser companies usually only update up to 2 versions behind. Then again if it was a different entity they probably would've made a version of IE 11 compatible with Windows XP.