A currency cannot exist by itself. Its value is relative, usually to some other currency. Criticism of fiat - infinite supply and under central bank control. Crypto is supposed to fix these two
You are correct that no currency exist in a vacuum and everything is relative. The dollar is strong because the possibility of nuclear (or at least very cold) winter in Europe.
Crypto isn't fixing the infinite supply, because contrary to their PR, cryptos are infinite. When you can fork something effortlessly, there is no limit in your existing numbers.