This is a fair enough argument but some goods and services you can only pay for via fiat. In fact most goods/services can only be paid for by fiat or credit. Inflation is going to be a thing at all if you are just selling the amount of BTC you need to make a purchase in fiat. I am guessing you mean exchanging for large amounts of fiat that you wouldn't be using in a few days time.
He has a point, we understand about it. Why we should sell the coin if it reach a good price instead of shopping spree using bitcoin at online stores.
But I agree that there are things that you can't buy without fiat money, it depend on which country you stay too. Not every country has many stores accepted bitcoin as payment even in the next 10 years.