So the money supply must generally keep growing in a growing economy. How should that be effectuated? For a long time, we tied expansion of the money supply into how quickly we could mine gold and put it into vaults. Given the exponential growth of the economy, and the distinctly non-exponential nature of gold production, it's obvious that wasn't going to work for long. So we have what we have now, with a central bank controlling the money supply.
That's the justification for the existing system. Are there alternative mechanisms that don't involve someone with their hands on the big money supply dial? Maybe, but I haven't seen any convincing ones yet.
Yeah, you sure wouldn't want the purchasing power of your money to increase, would you now?
There are economists out there who hate the idea of having a central bank control everything--they tend to argue for rule-targeting central banks. Examples include the inflation+unemployment target (the Taylor rule) or rules based on the circulating quantity of money (NGDP targeting). So some people think there are ways to have a currency that is simultaneously economically good and manipulation-proof.