This type of service needs Apple and Google support to go anywhere, given how restricted access to radio hardware and background processing is on iOS and Android, and they're clearly not interested.
Apple has even rolled back AirDrop functionality, supposedly because of people receiving unwanted photos (which I don't doubt happened, but changing the defaut could address that – just outright removing the option to receive from anybody seems wrong).
There's absolutely no reason we shouldn't at least have a P2P Wi-Fi based chat client preinstalled on every iOS and Android phone, with a default of being able to message only known contacts. I mean, even the Nintendo DS could do it in 2004!
I never used it, but remember the hype. It didn't get there by not working.