I outlined it above, it only makes sense. Humans have a tendency to get rid of the weakest moneys. If you were in Africa a couple centuries ago and had the option to trade with glass beads, which would lose all its value in the near future, or gold. Which would you choose? 
Same thing applies today. Why trade in Bitcoin if you hold fiat. Only once everyone saves in bitcoin and it sucks up the monetary premium laying around in real estate, gold etc and the price stabilises, then it should be used as MoE. It’s only justifiable to spend bitcoin if you earn in bitcoin and hold no fiat.