We estimate that by 2025, Signal will require approximately $50 million dollars a year to operate—and this is very lean compared to other popular messaging apps that don’t respect your privacy.
What business model? Why does a messaging app need to be a business? And again, how is someone who doesn't have service supposed to be receiving/sending messages? Makes no damn sense.
Basically all bittorrent programs include allowing a peer to act as a tracker directly.