Bitcoin would collapse under the load far before the entire US economy changed over to bitcoin, let alone the entire world economy.
https://en.bitcoin.it/wiki/ScalabilityAnd, assuming we did somehow make bitcoin about 100x more scalable than it is, sometime before the entire US economy transferred over to it we'd have bitcoin heists which made the recent mtgox hack look like pocket change.
Never thought of this. I guess that bitcoin was designed to be a small currency and can not grow that much. I don't think that is bad... I would call it realistic.