I don't think you can calculate the value this way.

All bitcoins will never be in circulation. It's like a stock, if you start selling a whole bunch, it will drop value quick. Apple market cap is around 600B$. Do you think that Apple represents more than 1% of all money worldwide?

If people hodl like they say. It could increase in value a whole lot. I believe it could increase more than the 15-20k you guys say.