I don’t really understand why or how this is different from your current trust model around email...?
To tout it as an alternative, or "breaking free" of WhatsApp, is to discredit one of the pivotal reasons that people use WhatsApp, which is E2E encryption.
That's it. Facebook could remove E2E today, and lose only a fraction of its WhatsApp users. Of course many little annoyances might mean that a competitor may step up, but WhatsApp is incumbent, and this is no longer about features or technology. It's all about the fact that your friends, family, and colleagues are using it. It can safely ride the network effect for years.
I welcome any protocol that makes chat something that is available for everyone with a computing device again — not just on approved smartphone operating systems.
1: Varies quite a bit per country.
WhatsApp simply won the race by being there early on and getting the critical mass. From there on out people are locked in, because everyone else is there.
Even in countries that had free domestic text messages (where the “free” factor was much less important), WhatsApp nailed group messaging better than anything else available at the time. Also, by using phone numbers they skipped the entire “create account / invite friends” stage, making for a much smoother bootstrap.
In Germany I'd say it's by far the most common chat client, even my parents use it.