You are assuming that the dollar and the euro will be rock solid currencies. With that assumption, you're conclusions are correct.
Is it very probable that USD and EUR will lose value and people will be looking for alternative money. When people see the financial collapse happening, they will be willing to learn how to use bitcoin.
Gold is pretty much useless, only makes sense as a hedge against fiat currency collapse. Yet the value of all the gold in the world is ~7 trillion USD. Bitcoin is ~1 billion. It's a gamble, sure, but I'd rather bet on bitcoin than on gold.
I'm assuming that the USD and EUR will be relatively stable for the next 10-15 years, yes... But if they did loose their value, why would people go straight to BTC? Most people are completely unaware of Bitcoin, and I feel that precious metals and physical valuables will be the obvious go to investment for most of the population.... That said, I am holding out for your aforementioned scenario!
There's no chance that these paper currencies can hold stability for that amount of time. There's already indicators that Bernanke may back off on stimulus even tho he hasn't expressly stated. There's a portion of his board that want to back off and this will affect the phony stock market that is propping up this facade of stability. I'd venture to say that being stable longer than 5 years is pushing it optimistically. When those in the market get their clocks cleaned, watch out in bitcoinland - the ship is leaving the port.