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.
The price of bitcoin is not a factor that determines supply. Supply is fixed. However, if people decided to sent their bitcoin to a burn address or for that matter lose access over thei rwallet, you can say the supply is down. Nobody knows the amount of bitcoins that are gone that way, and even a untouched address for 7 years, doesn't say antyhing.