The Gross World Product (GWP) is currently around 80 trillion US$. (That's a US trillion, a British billion.)

So, if today you managed to mine the entire 21 million BTC and replace all fiat currency with it, 1BTC would be roughly equal to 3,800,000 US$.

However, the GWP grows approx. 3% per year, making BTC worth way more by the time it's all mined should it replace fiat.

And, despite what we all wish, unless you had a hegemony that adopted BTC, you wouldn't replace all fiat. And, BTC being a pretty libertarian currency, I highly doubt most BTC proponents want a world government.