Any currencies value is based on supply and demand. We know the Bitcoin protocol is written in such a way to only release 21 000 000 in it's existence.
So no matter what happens with fiat, the value of Bitcoin will stay relative to it's supply {It's rarity} and how people value it.
Remember years ago people used silver and gold to trade, and they considered it valuable enough to use as a currency.. but then there were not enough
gold and silver to use for normal trade, so they created a voucher {fiat money} to represent it's value. These vouchers has been linked to gold for many years, until governments
printed so much of these "vouchers" that the gold in vaults could not represent it's value anymore. {So fiat money is not based on rarity of minerals}
Bitcoin just replace this "voucher" system, with the only difference being, it cannot be "printed" without any limits.. The limit was set, when the protocol was released.
This limit can be changed, but nobody would accept it, because it would de-valuate their coins they own now.
https://www.youtube.com/watch?v=EobPnLZiOo8