I don't think they're totally off-base: I haven't used my phone contact list for personal contacts for most of the last decade. It's just a collection of work contacts that I don't trust enough to add anywhere I actually talk to people.
I'd say at least 95% of smart phone users in the UK use WhatsApp. I think that's probably true of the rest of Europe as well.
Do you have multiple Discord servers or just one for all your friends?
Roughly how old are you and are you a student or in a job or something else? What country or region?
When you meet a new person you want to stay in contact with, how do you do so?
Do you use only Discord with friends or do you also add them on eg Facebook or Email or any other communication system?
In my world (employed, UK, middle aged) at work generally we use Slack (kinda like you’re using Discord in a way), shifting to phone numbers when you know people well for non work stuff.
Everyone else I meet, the assumption is to exchange phone number and use WhatsApp - exceptions would be iMessage or Signal sometimes. Or email or Twitter in business circumstances.
We had AIM and message boards pretty early, but no dedicated phone lines. By the time cell-phones became ubiquitous, we had cars and could just actually hang out. And by the time we all moved apart, voice chat services were good enough to just hop back to the old chat model.