Well, I don't know where parent is located, but here in Brazil they are pretty much right.
Everybody has WhatsApp... it's assumed that if you have a phone you have WhatsApp. A lot of financial transactions are conducted by doing a bank transfer and then sending the receipt via WhatsApp. The schools send essential messages to parents via WhatsApp. During lockdowns, stores were taking delivery orders via WhatsApp.
What's more, several cell phone operators' plans include unlimited WhatsApp, but only a few GB of other data... meaning when your data limits are reached you can still use WhatsApp for the rest of the month, but not i.e. Signal or Telegram. That makes is rather hard to convince people to switch.
So yeah, pretty damn close to critical infrastructure. I'm still going to delete my account and see what happens, but unlike the siblings here I can't disagree with parent, at least for Brazil.
[Edit] When I delete my account I'm going to lose old conversations, but that doesn't bother me anymore because I already recently lost them once when I switched from Android to iOS... WhatsApp has no way of keeping your history in that case. And otherwise there's really no friction to deleting an account, since you can always recreate it.