You do realize that what we call 1 BTC is pretty subjective. There will technically be more than 21 million BITS(pieces) of coins, the number of discrete units will be ~2.1*10¹⁵ I believe.
So the actual number of potential base units pr world citizen is (2.1 * (10^15)) / (7 * (10^9)) = 300 000 (correct me if I'm wrong here)
Of course I see where you're heading, but when the world gets to a point where every citizen uses bitcoins, there will probably something new, maybe something backed by bitcoins, since bitcoins are too valuable to be practical for daily use.