Where did you get that information from? Satoshi never said that. And this makes absolute no sense in my opinion.
This is so unrealistic, almost delusional.

BTC will not replace fiat currencies completely, if it becomes an internet money, a travel money, and a reserve of value and so on it will be amazing already.

Fiat currencies and traditional payment methods (such as wire, swift, visa, etc) have lots of limitation which our generation (most of us here are millennials I guess) don't accept.

For example, if I want to buy a digital good in Amazon.com I will be charged about 10% in banking fees and taxes. This makes no sense. While if I could buy with bitcoin, it would be ok.
There are also inflation problems.

However, fiat currencies certainly have their pros, just some of them:
- Cash is 100% anonymous, more than Monero.
- You can store cash in a bag and you know that you don't need even an internet connection or battery to spend it.
- The existence of hundreds of fiat currencies than a single currency (bitcoin) is important for diversification in the world. What if someday someone finds a bug in bitcoin code, or finds a way to break down cryptography or whatever? World can't rely only in Bitcoin code.

Let's no create a fantasy that bitcoin will replace all hundreds different Fiat currencies in the world. There is space for them to coexist, and we probably need it.