Considering the assumptions in my older posts quoted below remain mostly valid and that most new internet users do so on smartphones...
The last time I've estimated the size of the full PC universe, as you can see in the quotation above, I came to 2 billion machines. Let's assume it didn't grow any, just for the sake of simplicity, and that means 5.66% of 2x109 = 113x106 machines or, in other words, there's still a minimum of about 100 million XP users today, not counting the true POSReady and related machines, which purportedly don't browse the web, so that they don't get counted by netmarketshare.
So no, not at all, we're still very far from being the last half-a-score of XP users in the world!!!
... we should still have a desktop user universe of not more (probably somewhat less) than 2 billion users. Now:
3.19% x 2 x109 = 6.38 x107 & 2.51% x 2 x109 = 5.02 x107 ... to get to an estimate that minimizes our ignorance, let's take the geometric mean of those two values and we get 5.66 x107, which means there should still be some 50 million XP users today, which is not bad, considering we're 6 months away from POSReady 2009 EoS.