I understand that WhatsApp may be necessary to talk to businesses (because Signal didn't develop that, and I honestly don't think they should).
But what would prevent people from using WhatsApp to talk to businesses and Signal to talk to friends? I have been using multiple channels with friends forever: phone call, mail, email, MSN Messenger, Facebook, IRC, ICQ, WhatsApp, Threema, Signal, Slack, Discord, Matrix, ... What sucks is when I can't reach a friend. But I never saw it as a problem that I had too many choices to talk to them :-).
I don't really understand this "It has to have 100% of the market" stance. I don't want monopolies, I don't really understand why someone would say "this monopoly sucks, but I really want a monopoly so I won't ever change unless it is for a better monopoly".
For 1:1 conversations I think you're right. Having multiple channels for communication is fine.
Where it breaks down is for group conversations. If Person A won't use Signal and Person B won't use WhatsApp, you can't easily have group communications. And it only gets worse as the number of people in the group goes up.
In my experience, people who use Signal usually also have WhatsApp. It's really mostly that many people absolutely refuse to install Signal on their phone. Like they have all sorts of apps (including social networks that are sometimes downright malware), but they will fight against Signal for some reason I don't understand.
> (because Signal didn't develop that, and I honestly don't think they should)
FWIW, as far as I ever could tell, Facebook did this correctly: the only real thing is letting a business have an account without a phone number; they then provide the software you can run on your server to be a WhatsApp client, so all of your user's messages are then end-to-end encrypted to your business. Yes: later on they decided they'd get in the business of offering a "hosted client"--which meant that, technically, if you used that service, they could see the messages, which caused a change to their terms of service, as a blanket statement that Facebook can't ever see messages isn't technically true anymore, which Signal threw a ton of FUD at :/--but anyone could have offered that service before (and could right now also for Signal).
But what would prevent people from using WhatsApp to talk to businesses and Signal to talk to friends? I have been using multiple channels with friends forever: phone call, mail, email, MSN Messenger, Facebook, IRC, ICQ, WhatsApp, Threema, Signal, Slack, Discord, Matrix, ... What sucks is when I can't reach a friend. But I never saw it as a problem that I had too many choices to talk to them :-).
I don't really understand this "It has to have 100% of the market" stance. I don't want monopolies, I don't really understand why someone would say "this monopoly sucks, but I really want a monopoly so I won't ever change unless it is for a better monopoly".