I am not sure why you are speaking in hypotheticals. The economy of lebanon did collapse. And if you were holding bitcoin, you'd be much better off than holding that local currency.
Economies collapse all the time, around the world. And because the coin is a global market, it is insolated from local collapse.
> it really doesn't matter what sort of currency you use
But it did matter! Just look at the current situation, in Lebanon right now. If you were holding the local currency, you'd be screwed. If you were holding a global currency, you'd be much better off.