> If they don't have WhatsApp

Is this really likely? I would guess only 70 year olds don't have WhatsApp.

Seems more likely you forget the impoprtant phone numbers because you never enter them manually.

Nobody in my friend group has a WhatsApp and I'm a millennial.

Depends where you are, in most European countries, everyone including their grandmas will use Whatsapp.

It doesn’t have to be just for one type of user.

With iMessage, Discord, and Signal, I can't imagine a use case for me or anyone I know to use WhatsApp.

The vast majority of the world defaults to whatsapp. But yes, other than convention, any of these could be equivalent.

I don’t see why you would care for Discord over whatsapp except that it’s what you happen to use already.

WhatsApp is not universally used. Line, WeChat, and KakaoTalk are more common in East Asia

In the US, the only people I know who use WhatsApp are immigrants who use it to talk to friends and family back home.

lol not 70, no Whatsapp.