Its not just that, there will be way less than 21 million bitcoins for many reasons.
1) Alot of wallets have been lost, stolen or hacked, on purpose or by accident
2) Satoshi mined a huge load of coins when bitcoin started, and its unclear whether he will ever be back to spend them
3) Alot of people through laziness have lost coins mined in the early days
4) Its very easy to lose coins, especailly if you have encrypted/hidden them, if you happen to be die for example, your coins are gone in this case
5) Random computer crashes, your house being burgled, or fires can destroy bitcoins if you havent backed them up
6) Sudden amnesia can cause bitcoins to be lost
7) Any of a dozen unlikely possibilities can also cause coins to be lost
Ie, very few people will ultimately secure their coins properly, and those that do, may simply get hit by a bus tommorow. Therefore, over the next 10 years, most coins will disappear into oblivion... This means deflation, but it also means the remaining coins will be much much valuable as there will be no where near 21 million coins that are actively usable/traded....