it is highly unlikely that BTC become a single payment method worldwide, we shouldn't count on such scenario.
Let me remind you how this topic started:
We are talking about one thing, just how we can talk about how expensive is it to heat your house with garbage if you don't have gas, coming in and saying that is inefficient and we should use gas is not about the subject and scenario at hand. Stick to what this is about, not what if, can't be, it shouldn't, not possible, no way, let me tell you otherwise.
Imagine people in country A (this country already have its economy, production, import and export etc) decided to switch to BTC only. Since they are trading with another countries they have all production costs in BTC and they are selling everything for BTC while country B (which pays with their own money) is having continuously increasing prices. In the end, you only your products way overpriced in the world market just because you have switched to BTC.
Again, you're going outside the proposed scenario.
Simply put:
You produce 7 billion things of stuff x, 7 billion of stuff y, 7 million of billion z, all for 21 million coins in circulation.
Fast forward, the earth doubles in population, you produce 14 billion of stuff x, y, z, all for 21 million coins in circulation.
What do you think will happen to the prices expressed in BTC ?