Orphaned coins lower the amount left. That can add up, since it never goes down. Because of this, it will at some point reach 21 million. The key is to somehow put all of these coins in the reward without the used ones going as well. Is there any way to do so?
Just to use proper terms, 'orphaned' (as in orphaned blocks) is not the same as 'lost' (as in lost coins).
A 'lost coin' is generally a coin where the private key has been lost (like with a lost wallet.dat) and this is not recoverable. Such coins reduce the total number of bitcoins available. As Burt pointed out, this has been discussed in several threads on this forum, just search for it.
An 'orphaned block' is a block that is not part of the longest chain, something like that could happen if someone else mined faster and the network always recognizes the longest chain as the valid one. Orphaned blocks thus don't count towards the total coins mined, and the don't 'add up'. The only way to use orphaned blocks is to continue your own alt-chain (if anyone cares). Bitcoin would only care for blocks in the longest chain.