it has a permanently predictable rate of inflation
It’s volatility would suggest otherwise. As many others have commented here an in other places, the fact that supply is predictable is not enough, demand is a factor too.
Inflation rate means the rate of issuance of new coin. Its price is related to whatever free market forces decide the price of anything scarce such as gold, silver, fiat currency, etc.
In economics the word inflation refers to the rate of change in the buying power of a currency.
What you’re talking about is the money supply (I guess the Bitcoin equivalent of M2) which in Bitcoin is for some reason increased at a hard coded (decreasing) rate.
