Have you ever paid for a sandwich with pre-paid phone credit? In places where it's used, it's surprisingly usable.

Yes! But, you do get some bonuses with cryptocurrencies. Access to credit, ability to build an account history, etc. At the very least cryptocurrencies are a superset to M-Pesa etc. Plus some other geo-political perks like not being centralised around Chinese technologies (M-Pesa might not be, but its competitors are & underlying mobile tech is predominately Chinese).