Some people will not change their OS until their computre physically dies and it becomes too expensive to repair. Then, and only then, they will buy a new PC and it will have the current OS on it.
Companies will transition as budgets allow.
MS has (or will soon?) provide only security patches for XP. New third party software will not be developed for XP.
Like an old soldier, XP will fade away.
There will still be a few collectors who will keep a copy running on a Celeron 500MHz box just for the heck of it.
And we will all talk about the 'good old' days when XP.....
Hey! Remember the good old days when Windows 98 came out and memory expansion cards were $300...