I agree that the USPS is a good use case for more electric vehicles. I'm not on board with the "most USPS vehicles should be electric" crowd until it's studied. I think there's an amount of them that should, and amount that shouldn't. Either way the switch over will have a high up-front cost, which should be paid back over time with savings on fuel. Given that the USPS has significant lack of control over its finances, that's a point of consideration.
I think it's more complex than a lot of commenters will contemplate, and is worth actual study with real-world use cases for the USPS in particular. Do I think it will work out that a large percentage of the USPS fleet should be electric? Yes.
My experience in talking to folks who have EVs tells me that the posted ranges don't hold up in real-world conditions (especially in cold or hot months) and that charging is a challenge when not at a "base". That "somewhere with a good charger" is likely to be an issue in many rural areas.
Your point about moving the existing ICE vehicles to the rural routes as the new EVs come into service is a good one.