But that's the thing: The primary use cases for something like this are the opposite of crowded urban areas, where infrastructure-based networking makes a lot more sense.
I'm not trying to save on data fees; I'd just love to do low-bandwidth peer to peer messaging with people nearby without any network around.
What if, for example, low-frequency 5G bands were available to such P2P applications as a secondary user, similarly to 5G WiFi and weather radars? If there's a network there, use that; if there isn't, do P2P!
You ever try and have a conversation over APRS text messages without even digipeaters in a crowded urban area? Good luck getting through.