Mobile maintains good battery life mostly by being idle almost all the time, and P2P can’t work that way. Even P2P with central server assist is hard on mobile.

It could if we’d never broken the full end to end principle and put stateful firewalls and NAT in front of everything, but that ship has sailed. All net connections require keepalive or a firewall will close them. This is even true these days in the cloud for the most part unless your machines are “naked” VMs directly on the globally routed Internet.