Over time bitcoins will be lost, and by lost I mean forever. Inevitably wallets will be corrupted or deleted and can't be recovered. Inevitably someone with bitcoins will die and their heirs will not know how to recover them, if they know of the coins at all. In theory this loss could add up significantly over a long period of time. With the total number of bitcoins capped what are the implications?
Forever is a very long time. As the number of "lost coins" goes up, the chances of people opening a new address and finding a pleasant suprise within also goes up.
As the number of bitcoins drops, the value of each bitcoin goes up, so people hold less and less bitcoins in a single place, and so each time bitcoins are lost the number of bitcoins goes down. In addition, as the value of bitcoins goes up, and as knowledge and protocols increase, people will lose less and less of their bitcoins (They are more careful with what they have, since it is worth more, and better layers of protection are invented and used to prevent total loss of bitcoins.) Based on unscientific polls of users of this forum, most bitcoiners are in the 20-35 year old range, these young people do not think about death and wills and providing a means for people to access their bitcoins if they die. However, as the bitcoiner population ages, they will (on average) think more about these things and many will put into place some mechanism for the coins to be passed to their survivors.
Just consider the math behind what you are suggesting. Let's say we have 100 units of currency, and we loose 1% of the outstanding amount every year. After 100 years we will still have 36.6 units of currency. So all the prices will only be about 1/3 as high, and we have had 100 years to be accustumed to the change in price.