Bitcoin is not yet fully payment system like fiat.
It is.
Bitcoin is high volatile and common people of Afghanistan would not like it. Well devolped countries should choose bitcoin for payments purpose than off course some small countries will also adopt it.
That's the matter for what they see in bitcoin. Bitcoin's purposely being used as both, investment and payment. But mostly today, we're all holding it and treats it as an investment and that's what's being said on the original post because it cannot be confiscated by any government unless you're the one surrendering and voluntarily giving it. And for the developed countries, they're not obliged to do it, but as long as there's no law about it, people living in those countries are free to own, use it for payments and as a store of value.