> Bitcoins [...] _could_ become as universal as PayPal is today
Putting aside numerous other issues, bitcoin.org states "The total eventual circulation will be 21 million bitcoins".
[EDIT: DanI-S pointed out that bitcoins can already be subdivided. So this was switched from "there are too few bitcoins to support even American e-commerce" to "bitcoins are not valuable enough to support even American e-commerce". The number of fractional bitcoins is still an issue, but less immediately.]
At the current ~$0.70 / bitcoin, this means that every American will be able to have ~$0.05 in his or her electronic wallet, once all bitcoins are generated. Assuming that the rest of the world does not participate at all and that bitcoins are evenly distributed.
Sure, you could imagine an instant dollar-to-bitcoin-to-dollar conversion at the point of payment. Or you could imagine a bitcoin2.org that generates more coins. Or you could hope for a massive surge in the value of the bitcoin.
I'd put my money on Paypal sticking around, though.
The obvious hole in your chain of reasoning is that "$0.70/bitcoin" would obviously change when every American attempted to fill his wallet with bitcoins.
Bitcoins probably won't take off (cf the failures of all the other alternative digital currencies so far), but the whole number of bitcoins is no more a roadblock than the limited number of ounces of gold in the world was to getting everyone using gold as currency. The problems basically lie elsewhere.
I'd put my money on paypal sticking around, but I'd also expect the volume on the Bitcoin/USD markets to increase as the value climbs (presumably from rate of new bitcoins from mining declining). I think we could see services using fractional bitcoins behind the scenes and USD (or euros) end to end.
I bet you could make some money facilitating lower fee transactions backed by bitcoins.
But yes, it probably doesn't make much (commercial) sense right now.