Zoom did ONE thing amazingly well. It's was super easy to get people into a call. Click link in email --> be in zoom call. No fuss, no frills, No extra buttons or menus for Grandpa to accidently click and get lost in. More importantly no account needed.

Even starting a new call was easy. You could email the link to someone from in the call you were in. scheduling calls had a similar super easy UI

As for replacing other options, it wasn't just Discord, lots of other video conferencing software exists, and was in fact already being used for b2b (Cisco).

Have you ever tried to schedule video call event in discord with people who haven't used it before? I tried to host virtual Halloween parties etc in 2020. It worked but it was very clunky.

In order to have a specific guest list I had to create a new server. Then email out a server join code. People trying to join were forced to make a discord account if they didn't have one. And once in the server since people didn't know how to actually join the video call portion and how to turn on their video in the voice chat.

On the flip side I saw people trying to use zoom for a virtual conferences... Which was always a disaster because zoom doesn't have the tools for that. Meanwhile discord or something similar would have been perfect.