I'm with you, measuring in fiat is a crazy way to look at the world because its supply increases arbitrarily. On the other hand, how will we measure growth in Bitcoin terms, once fiat is out the window? In your calculation above, you divide 900 trillion by 21 million, which makes sense for right now. But once there is no more fiat, what is the denominator? I know the meme, "infinity divided by 21 million", but concretely how will we measure e.g. economic growth in a Bitcoin denominated world?