Bitcoin is a dependent currency. You are generalizing it as bitcoin is only for USD but the fact is not. BTC is convertible to all fiat currency around the world. It is a standalone coin, it does not depend on any value of fiat currency. The opposite statement was, the value of USD>BTC will depend of the current value and demand of BTC in the market. Because that is the fact.

You can research it for more informations which may enlighten you.