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.
Without really understanding or having thought through things fully...and if the basic premise of the system stands the test of time...
I kind of envision a situation where the coins in the first chain becomes backing for further chains. Those who hold coins from the first chain would then be able to back a given branch which is managed to their liking, and people could choose which branch to use for similar reasons. I theorize that if both the 'funders' and the 'users' of a currency need to arrive at a mutually agreeable management strategy, more equitable end results will fall out.