Some things take a long time to fail, but will undoubtedly fail. There are numerous examples of failed fiat currencies in modern times and ancient times. Just because dollars, euros and yen survive now, doesn't mean it will survive forever.
The core reason for this is simple: humans are bad at decision making in groups. Economic policy becomes sensitive to election cycles, populism, partisan agendas, etc. Every economic recession becomes a dire problem to be solved with state intervention. Every economic expansion becomes evidence of good policy decisions that emboldens some economic theory, convincing people that good times can continue indefinitely with the right policies.
The rules for that govern economic systems SHOULD be concrete, they should not be subject to human opinion so frequently. But we find ourselves pushing the edges of what amount of debt is acceptable, or how inflation is even measured.
The rules for money should be transparent, mathematical, algorithmic.