Even if the price of bitcoins goes in millions still fiat will not become worthless as we need fiat to convert our bitcoins as even in future I don't think that we will be able to spend our bitcoins at each and every place.
We do not have to spend our bitcoin in every place but I can ensure that it will be very easy for use to spend crypto in the future modern world. Bitcoin is the leader of cryptocurrency and it will affect the world a lot in developing and improving the cryptocurrency and the blockchain technology