A reproducable build means you can get the same source code and compile it, and it will be identical to the published image. This is important because otherwise you don't know if the published image actually used some other source code. If it used some other source code, the published image might have a backdoor, or something that you can't find by reading the source code.
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".
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.