Thank you @superskillz for this very elaborate answer.
The math looks solid and its very helpful :)
It looks like it could take some time to reach 40 billion though, so it might not be the best value investment at this moment.
There is also the volatility of the markets to take into account when using these debit cards.
When buying something (lets say a cup of coffee) the card will withdraw a certain amount of lets say BTC. This debit transaction will happen immediately and in real time. Which means that you end up paying the going exchange rate of BTC, that could mean huge uncertainties as to the actual price of the cup of coffee.