I think that you are letting an aspect of bit coins pass you by. Because of the rigid quantity of bitcoins, the more success it finds the less it will circulate. It's a deflationary currency, the more widely accepted it becomes the less of it will circulate. People will choose to hold bitcoins speculatively (you could argue this is pretty much the case already) rather than use them for casual transaction. I don't recall who's theory this is, but it's basic monetary theory. When there are two forms of currency people will horde the one that is more sound. A deflationary currency is always going to hold it's value better than an inflationary currency so people would always choose to hold their cash holdings in bit coins and spend USD or whatever else they use. I think this is going to be a problem for bit coins.