Bitcoin has no physical presence. What if we just stop believing in bitcoin. I mean every country has the equivalent of gold to their fiat. Bitcoin has no gold or physical value. Can this ever happen ?
Fiat money is not backed by gold either. The definition of fiat money (copied from Investopedia): "Fiat money is a government-issued currency that is not backed by a commodity such as gold." So technically we could just stop believing in fiat money, too, but once a type of money starts to have economic significance, it's not quite so simple.
For bitcoin in particular, part of the value comes from the limited supply, therefore demand is a big driver of the price. It's the algorythm behind it that ensures it is valuable.