Fixed monetary supply might be one thing in theory, but fixed credit supply is the death knell to an economy. And the two are usually complimentary. Monetary supply can fill in the potholes of weakening credit supply when the latter contracts - and it inevitably will when the private issuers that generate those debt contracts pull back in a recession. If you have your hands tied to some constrained supply like gold or bitcoin, like what happened to the central banks after World War I and into the Great Depression, you end up backed up against a wall.
That said there is certainly something that's personally appealing about an asset that's guaranteed to maintain the supply side of the equilibrium. As an investor you only have to consider demand. But for any kind of widely used currency that backs an economy it's going to be problematic.