The thing with WhatsApp is that besides the largest user base (most important reason IMHO) it just works very well. I don’t know about Signal as I’ve briefly used it but why would anyone switch apart from us?
You can’t just convince the majority of the users with privacy arguments or even hypothetical extra features from Signal. Only if WhatsApp would introduce something really annoying (huge ads, fees, constant technical issues) people would start to move. But even Telegram which is so much better IMHO (albeit privacy by default isn’t better) will hardly be #1 ever if nothing of the above happens.
They charged $1/year “on paper” but never actually collected it. That would have been plenty of revenue for any normal people but they got greedy and realised their user’s private data was worth much much more.
"They" = Zuckerberg & his minions after acquiring Whatsapp and falsely promising that no, they wouldn't dare mining Whatsapp user data or -god forbid!- integrate WA's backend into FB's.
At scale, that dollar will go very far. Unmetered bandwidth is cheap if you look beyond the cloud providers and the majority of calls can be established directly (in fact WhatsApp does use UPnP to map ports presumably for direct connections) so you'd only ever use that bandwidth for texts/transient media uploads and the small percentage of calls that can't be established directly and need to be proxied through. WhatsApp doesn't store media long-term so storage requirements are also small.
How does P2P work if I send a video to a large group? It must be hosted centrally, unless my phone is constantly keeping track of who hasn't had the video yet, and uploading it.
The participant with the most bandwidth (and I guess since it's mobile devices, other factors can be taken into account like battery level, etc) becomes the host and receives video from everyone else, multiplexes it and then streams to all other participants. Unless you have 10+ people on the call, a typical home connection is sufficient. That's how Skype worked fine for a decade before Microsoft screwed it up.
But even if we assume WhatsApp should be multiplexing the streams on their end, unmetered bandwidth is cheap and again we aren't talking about 4k streams and hundreds of participants per call.
You can’t just convince the majority of the users with privacy arguments or even hypothetical extra features from Signal. Only if WhatsApp would introduce something really annoying (huge ads, fees, constant technical issues) people would start to move. But even Telegram which is so much better IMHO (albeit privacy by default isn’t better) will hardly be #1 ever if nothing of the above happens.