yes 21 million is the upper limit, but whether 15 or 21 million, that is a very limited supply of bitcoins for this entire century.
If it does take off as more and more of a payment system, well I think everyone can figure out the price would go up - way up.
BYSD
Actually there is only like 15 million btc in the world right now. 21 million is the max that can be mined Which will not happen for another hundred years.