Something yet to be brought up:
If coins were lost, only the owner would know. If he told others, there would be no way to verify it.
This means that you can never know how many are lost, so you can never truly change the value of the rest of the coins as if they didn't exist.
The point is there isn't someone who has to 'value the coins'. It happens invisibly.
If a lot of coins are lost, as a direct result, less coins will be supplied on the market, and if the demand stays the same that means the price will go up. It indirectly influences supply/demand, even if just on the long term.
If there were a central organization that had to keep track and value the coins that were in circulation based on given dollar figures and the amount of non-lost coins, you would be correct. But that's not the case.
It is correct that lost coins will not directly influence the (perceived) value of the rest of the coins - that is just an easy way to explain the concept. Lost coins will however influence supply/demand in the long term.