Yes, it should be the value or the price of the bitcoin should go down not the quantity of bitcoin itself. If you have 1 bitcoin it would be 1bitcoin forever. Only its price would fluctuate. As to the exchanger you are saying, I have no idea on what you are saying, maybe you should state the name of it.
No matter what happen bitcoin amount in the blockchain will never decrease in quantity. It may be not spendable but it still reflect in the blockchain. Those who lost their bitcoin wallet together with the passphrase and private keys with it and don't have backup will not be able to move their bitcoin and lost it but the amount is still counted in the total amount that can be seen in the blockchain.