As of 2009, we have some random guy on the Internet estimating $4.5 trillion as the value of all paper and coin around the world. That's a pretty decent number to use if you're looking to imagine replacing all fiat money with Bitcoin.

One big problem: there would be more demand for "cash" transactions since Bitcoins are more convenient than physical cash.

That is, due to convenience factors, a lot of us like monetary transactions that don't require the existence of any paper or coin. If we all switched to Bitcoin, how many of us would transfer BTC to a bank for the promise to pay back?

Anyway, $4.5 trillion divided by $21 million is $214,285.71. That said, that's in 2009 dollars with the 2009 world economy, and a lot of assumptions. Also, with a deflationary currency that is far more convenient than paper and coin, its value would likely be far greater than that if every single human being wanted to only use Bitcoin and have all financial transactions denominated in BTC.