It would be great, but I doubt that Microsoft will do something like that. Most likely Win XP users will get "only" security updates an service packs for IE7, Vista users for IE8 and only Win7 users will get the latest IE.
That being said, I can't see this as a big leap forward, since large majority of users will not get the latest browser update.
What large majority of users still use XP? I'd be surprised if winxp was 50% let alone 80%. It's official there are more 7 installs than Xp so no majority let alone a large one.
If you want updates upgrade to win7, xp is over 10 years old. Ie8 already runs fine on xp.
I'd be surprised if a great deal of other businesses are in the same situation. Outside of technology companies I haven't seen many running Windows 7; there isn't a strong enough business case for most people yet.