What has actually changed in the last 13 years regarding Whatsapp? Video. And I believe that's the reason why anyone hasn't actually challenged them regarding messaging: you can build a similar application with similar features with a rather small group of people (not saying it's easy, but it's feasible). But handling those pentabytes of bandwith shared every day? Actually _promoting_ the use of DIY video as the preferred communication media? That's something you can't do as an small shop. And that's, I think, why you cannot compete.
I decided to quit Whatsapp, which in Latinamerica is quite an outrageous move: that application is the communication channel for EVERYTHING: all families, all schools, all neighborhoods. I did it because I think Meta's main metric is actually hostile to their users: they want as much of your time as they can get from you, and they'll use are sorts of psychological weaponry to keep you inside. They were actually vocal about it in the past. There's zero reason to trust them. But why is it that no one has come up with a true alternative (although props to Signal)? Well, there's the network effect, for sure. They also employ very good engineers. But I believe the true reason is scale: it didn't use to be that way, but infrastructure costs are now inmense.