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.
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.
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 the fact that there will be always less available, and $ 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).
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://bitcointalk.org/index.php?topic=86894.0So 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.
There's also another scenario: once one government will see Bitcoin can't be stopped, 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 SLOWLY, SLOWLY get away from the scenario, because it will be less and less accepted in venues.
Zimbabwe did not choose a currency with a very limited suply. They choose dollars. Rest of what you said does not make sense at all. Nobody will sell a single bitcoin if bitcoin seems to be the dominant currency. Give me a single sane way bitcoin could replace fiat.