Your estimate is valid in ideal conditions:

• all payments are equal and evenly distributed in time
• there are marketmakers who immediately absorb all supply
• arbitrage between fiat pairs caused by selling can be settled immediately

In reality there will be orderbook that will hang much longer than 2-5 seconds needed to settle transaction. And there needed much more XLM on orderbook than average transaction because there will be transactions much higher volume than average.
Conservative gesstimate is 10x average transaction which leads us to 2 billion worth XLM.

Other approach is:
Just imagine you have one more fiat currency trading to all other currencies on forex.
It gives enormous liquidity to that new currency, hence gives it big value compared to other currencies. The price of that currency will be proportional to price of basket of all currencies, which gives it even more value given limited supply.

XRP and OMG all plays this field and expecting enormous gains even from current valuations.
Of course they can not achieve it, but their followers evaluate it from "world currency" model.