Can you tell how it could replace fiat? I se no possible way that could ever happen. If fiat should fail there is no way to buy bitcoin with that fiat since noone would sell and there is no way of getting bitcoin spread around without everybody buying it.
It's not the first time a country replaces its own currency with another one.
Latest example is Zimbabwe:
http://www.thehindu.com/news/international/in-zimbabwe-1-will-cost-35-000000000000000-local-dollars/article7309953.eceI don't know how this worked in the past, but seeing Bitcoin is not being possibly regulated in any way, it will probably go that early adopters like us are paying nearly nothing for it now, while in the future the price for one Bitcoin will increase considerably. Or better: exponentially, as we are just a small fraction of world population.
Now, this doesn't mean a person will have to buy always 1 BTC: that's simply impossible, also because there's only 21 millions.
So it's already in the blueprint that people will use mBTC or even smaller amounts like uBTC will be used for everyday exchanges.
Now the Bitcoin market cap is over 4 billion $.
This will continue to grow, up to a point where the Bitcoin market cap and the $ in circulation will be the same.
At that point, not only Bitcoin will grow in value due to increased market request, but $ inflation will begin to become unbearable.
The last persons on the planet buying Bitcoin, will buy it at a value that could well be 1 BTC = 1 billion $ (of todays value, in future, $ inflation could change that A LOT, it could be trillions).
But those persons won't be buying 1 BTC, they will buy a fraction of it, like 2 mBTC in example, or some uBTC in exchange of some thousand dollars:
https://bitcointalksearch.org/topic/how-much-is-a-mbtc-equal-to-86894So don't think that "one day there will be no $ to buy BTC, so that's impossible".
Think that 1) everything is possible, but even more, 2) replacement will happen slowly, more probably everybody will have some $ together with BTC for a long time yet.
Shops will accept BTC and $ for a long time.
But one day many shops will just want to cut out $ and live happy with BTC.
There's also another scenario: once one government will see Bitcoin can't be stopped, and its fiat currency being overwhelmed in inflation, it will declare its fiat currency obsolete and make it mandatory to replace it with Bitcoin before it goes out of course.
That will be probably the point when all fiat currencies will collapse and Bitcoin price will boost in a completely unpredictable manner: nobody will want fiat currency anymore, so from that point on, fiat currency will get away from the scenario, because it will be less and less accepted in venues.