Why are people paid in dollars when they want to eat bread? Because the dollars are a medium of exchange for exactly what the people want to consume, and at some point we want to consume what we have produced. You save to be able to afford consumption in the future.
Saving in a depreciating currency means your future purchasing power is diminished year after year by the tax of inflation.
A currency that isn’t being debased will retain its purchasing power per unit value, and if productivity in the future increases, it will thereby buy more than it could have in the past.
“Saving in a depreciating currency means your future purchasing power is diminished year after year by the tax of inflation.”
That does not explain why people owning appreciating bitcoins exchange it for dollars which are depreciating.