The problem with WhatsApp is not that users are locked in. You can install as many chat apps as you want. WhatsApp has no exclusivity over its users. The problem with WhatsApp is that it only has one alternative. In terms of chat apps that use the signal protocol there's two: WhatsApp and Signal. And Signal is not quite as good as WhatsApp. It's certainly much improved but it's not there yet.
The vast majority of users don't care about the underlying encryption. That's a problem, in a sense, but the core problem here is the network effect. That it's the natural behaviour of people doesn't make it not a problem.
The network effect works both ways: if key people in my network are convinced to switch, other people will follow.