It is at the whims of whoever decides the interest rate.
Which is precisely what makes it a fiat currency - its value is determined by a government entity.
> A US Government 10 yr bond currently yields 0.91%, and has been falling consecutively since its peak in 1981.
You know there are other fiat currencies right?
> not sure how you determined there is much utility in using it for saving.
"store of value" doesn't mean simply saving, it means the asset doesn't depreciate. The alternative to a fiat currency is to hold your goods as is and barter them, which relative to a fiat currency is a bad idea. Your cow eventually dies, your milk eventually soils, your tree eventually dies, etc. etc. In other words those assets all eventually depreciate to zero.
- Medium of exchange
- Store of value
Fiat currency has utility for both. If crypto was more widely accepted by merchants (thereby satisfying bullet point #1) then this certainly changes the game. But it's not.
Comparing both in their current capacities is disingenuous.