What would be the value of Bitcoin if coin generation was complete and Bitcoin was strongly accepted today? Bare with me for a moment during this thought experiment. Currently, there is about $1.18 trillion US dollars in circulation at the moment. With that said, my one dollar bill currently represents about 1.18 trillionth of the US economy. 

It is estimated that the last block to generate coins will be generated in the year 2140. At that time, there will be about 21 million bitcoins in circulation. And thereby, my one bitcoin would represent about one 21-millionth of the bitcoin economy. 

Now let's say the following were true today:


Bitcoins have been completely generated
There is about 21 million bitcoins in circulation
There is about $1.18 trillion US dollars in circulation
Bitcoin is as widely accepted as a form of currency as the US dollar
The strength of the US dollar is on par with Bitcoin


Due to the large amount of circulated dollars compared to the amount of circulated bitcoin, can we assume that 1 BTC would be similarly valued at one 21-millionth of the $1.18 trillion circulated US dollars? This would equate to around $56,000. I know a lot is being assumed here, but does anyone see any major flaws in this logic?
If it so happens that bitcoins are as widely accepted as USD are today, then yes that's a roughly reasonable estimate. 

You might enjoy the following link for some more thoughts along these lines:
http://falkvinge.net/2013/03/06/the-target-value-for-bitcoin-is-not-some-50-or-100-it-is-100000-to-1000000/

(One side note: just because new bitcoins are slated to stop being created in 2140, that doesn't mean no more new blocks will be created. Blocks are essential to record transactions, and they'll continue being appended to the blockchain even after the bounty drops to 0.)