XP will be supported for about as long as Windows 98 was. So for a good three, maybe four years after Longhorn is released (longer than 98; but accounting for the longer shelf life as main Windows of XP) - at which point Blackcomb will be out anyway.
Longhorn brings in all the numerous qualities I listed below - and only from what I have seen in the Alpha and from Reviews on the Alphas other people have got. Betas are always vastly superior to Alphas anyway; so nobody knows.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.