If you really wanted to learn more instead of just preaching your preconceived ideas, then you would start with an open mind instead of a fixed premise that "fiat is doomed to fail". That already shows you are an extremist and likely uninterested in a rational conversation.
That is not true at all. I say that they are doomed to fail in the long run because your purchasing power disappears year after year. You said that there are currencies that have "lasted for centuries" but just because they "lasted" it does not make them good. You can keep a person on life-support for a decade but what kind of quality life is that?
Back to the fiat currencies that "lasted centuries". If your grandparents left your parents a nice sum of 100.000$ back in 1920 and your parents left you the same amount, how much purchasing power would that money get you today? Back in 1920 you could have built a house. Today maybe you can buy a bicycle with it. How is that not a failure?