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.
It hardly needs to be more scalable, it's never going to be the new way of small purchase transactions like VISA, but I think the larger investors will trade the big dollar amounts more frequently than the average McDonald's purchase. There is just too many people using the US dollar to make a serious dent, but you could probably pay off your VISA with bitcoin money. I think people will be using bitcoin as a tax-free bank more than a means of payment.