In-game voice chat has always been hot garbage on nearly every game that I have tried in comparison to 3rd party, Expecting the standard to rise for games when there is very little incentive to do so (you are not going to get sizably more players on good voice chat) and at the same time expect it for not just one game, but all of them?

Also why would one want to readjust voice settings for friends on each and every platform? I don't want X amount of configs that I have to adjust for my laptop/desktop and/or when the games inevitably undergo a major patch and decide to hard reset all the settings for no discernible reason.

That isn't even touching the fact that discord is not only dominate in voice base services, but it is aggressively expanding simply as a messaging service as well. Channels of permanence, aggregation of content and communities, high powered bot services, Discord isn't just games anymore. Server personalization with Nitro features is definitely a great play and imo this take severely underestimates the importance of communities outside the gaming sphere.

edit: Check out a bunch of major subreddits and they will have Discords, people are already cutting out FB and a combination of pseudo-anonymity and personal community feel and you have Discord taking a slice of FB groups, hell I recently pivoted my music chat-groups to Discord, I don't even see why they won't eventually pivot to take bites out of Slack (I would assume they already are even without doing anything serious), hell for more certain tech-oriented groups it serves as the official platform of communication due to ease of accessibility and moderation tools.

>Once upon a time, games like World of Warcraft and Second Life included their own voice chat systems directly within the game.

I played WoW near launch in 2004, all the way through 2017. Voice chat was not an early feature of WoW - this was not added until much later (I believe 2016/2017)

Pre-Discord days were predominately Vent, with Mumble gaining popularity a little later on. Many guilds also used Skype. Up until the end of my playing - no guild I knew of used the in-game voice chat.