Bitcoin is already the world reserve currency.

I have some local US Dollar Currency on hand to buy my everyday walking around items.

I did the same thing when visiting Mexico in years past. I kept my long-term assets and savings all in US Dollars but exchanged my US Dollars for Pesos for walking around money.

I do the same thing now in my home state of Colorado... My Bitcoin is the standard and my local US Dollar Currency is the inferior currency I use to spend around town around.

No need for 100% Bitcoin adoption to get to this point.