Re: Use case
In India, the UPI infrastructure covers your points a, b, and c for domestic transactions. No one takes a cut, not even a small fixed amount, and it is instantaneous -- about the same speed as SMS or at worst a few seconds more.
(International transactions are not covered by this of course)
On point c, much as I hate the "your papers please" nature of India's Aadhaar, not to mention all the security issues it brings, it *has* helped poor people open bank accounts etc.
Also on point c, I do not think cryptocurrency enables the poor any better; most of them are illiterate, and you're talking about a system where **techno-literate** people regularly get scammed out of their savings, with no legal recourse because "immutable".
In short, I'd say "catering to poor people" is a particularly strong reason **not** to go for a cryptocurrency.
Point d is of course completely out, though most people don't care. I've found places where they discourage cash because it is convenient for the merchant to be paid digitally -- which would not be true if any of the banks in between were taking a cut.