Yes that's exactly what central banks are doing. Yes it can be easily manipulated as you can see in Zimbabwe or any of the failed economies of the past.
It's not looking good? Inflation is in the low single digit percent depending on where you live. That's pretty stable. That's pretty good. That's why you like dollars and obsess over the dollar price of Bitcoin.
Your fixed money supply will lead to wildly swinging prices and kill any business quickly.
Central bank fiat is the best we have until you come up with a better price stability mechanism but you have to have one.