What about if China suddenly asked for something in exchange for the tons of american paper currency they have accumulated? It is unlikely anytime soon, but they will only take it as long as it has value. So the more you inflate, the more you have to send to China to maintain the American lifestyle.
Our economy is becoming less rugged over time. A large shock becomes more and more likely to significantly disrupt our system. There is no redundancy or backup operations. Our cpu core's are all at 120% load.
<sarc>Also, its a good thing that stuff like population pressure and infectious diseases are a thing of the past (two of histories best methods for shocking a political/economic systems). You know, because the global population is under control and our climate is capable of withstanding everything we throw at it with complete aplomb. </sarc>
However when everything is going well, Keynesianism works well. I will give it that.