We use WhatsApp by default because of the network effects. FB have no power over Apple here because removing it from iPhones would kill the only reason people use it. The switch to (most likely) Telegram would happen overnight.
I agree that the messaging platforms are not all powerful but they do wield significant power. iPhone is famously good for social media because Instagram and Snapchat camera integration sucked and that was something people consider when buying a new phone.
So there must be a point where switching to a different phone to improve communications is less painful than making all your social circle switch platform for you.
Only time I use WhatsApp is when I’m traveling overseas.
At least taking the example of India, if Whatsapp were to stop supporting iOS, then people would still continue to use Whatsapp in their Android phones.