Let's assume that there is a non-zero probability that people lose their bitcoins, e.g. by forgetting their password. The amount of bitcoins these people possess will become "unusable" but still remain in the blockchain and contribute to the maximum quantity of 21.000.000 bitcoins.
Over time the amount of "usable" bitcoins would shrink more and more as people keep losing bitcoins. Will the amount of "usable" bitcoins eventually approach zero? What will happen then? Is there a mechanism to somehow recover lost bitcoins? At the moment I don't see such mechanism.