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 network effect works both ways: if key people in my network are convinced to switch, other people will follow.
And yes: users are locked in, because your whole social network is expected to be on WhatsApp. 1) you need to convince others to find you on alternatives (every single person even people you just met, you have to explain what&why; also: workplace, clubs, etc. or leave them). You certainly aren't going to convince everyone 2) you will miss out on a lot: people are being invited into groups, but they can't invite you anymore to just join; people create groups on the messenger they use most often, because it is more convenient for them. In summary, if you have a active social life and grow/grew up with it you will have a lot of friction if you really want to get rid of it. That is what I would call locked in.
Even this recent spate of "deplatforming" events can't seem to sway (a certain subset of) the tech community towards the idea that some amount of regulation can be a good thing.