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!!!