I think we should think of which requirements should have a currency in order to be suitable as currency to be used within X’ payment system.
So this is what I come up with at the moment:
a) it must be a currency accepted in all countries where X is present or want to expand to.
If that’s not possible, it must accomodate the countries which are most important for X, e.g. where most X users are located or where X wants to expand.
In other words, a currency with the least regulatory issues.
b) it must be a currency with a stable value.
The more stable a currency is, the better it suits as a means of payment.
Between a volatile currency whose future purchasing power is unpredictable and a stable currency, people use the latter as they don’t like risks: people look for predictability in choosing a mean of payment.
c) scalability.
X has more of 500 million users / month.
It must be a currency running on a network allowing thousands of transaction per second.
d) low transaction fees.
So, which currency suits these requirements?