$650bb / ($34.72 * 12 months) = 1.56 billion users.

That's far larger than the population of the USA (unclear to me if that 650bb number is global or USA only) but by sheer scale this is assuming that these companies can collect that fee from a global customer base - including users in developing economies, EU, China, etc. and after the middleman fees are accounted for.

The comments in this thread seem to be thinking within the context of 'the poorest in their nation'. This calculation assumes collecting this fee from among 'the poorest in the world'.

Sure, 1.56bb users could also be interpreted as 'the wealthiest 20% of the world'. But the tail is especially long on this curve given how wealth is concentrated in a small percentage of the global population (1% of users have 50% of wealth).