There is not enough bitcoins for everyone and that is why it's impossible for bitcoin to substitute the cash economy...
Just think about this:
A pre-defined schedule limits the total number of bitcoins so that they gradually approach a total of 21 million (ignoring those that have been lost through deleted or misplaced wallet files)
Total bitcoins: 21,000,000
Total humans: 7,360,837,156
That mean 0.002852 btc/human
but the total humans number is going up in the time. There is not enough btc for all.
Total
satoshi: 21 000 000 * 100 000 000 = 2 100 000 000 000 000
Total humans: 7,360,837,156
Satoshi/human: 285 293.636
By way of comparison, in 2006 there were estimated to be 16000 tonnes (=16 000 000 000 grams) of gold in the world (that's all the gold that had been mined, ever, up to that point), which would allow for 21.737 grams of gold per person.
I'm not too worried - I think this is sounding like a nice problem to have!
Edit: I misremembered by a factor of 10: it's
158 000 tonnes, not 16 000. Even so, 217.37 grams per person... I'm still happy with Bitcoin!