The absolute largest group of players in any game stuck with Mumble. That would be The Goonswarm Federation in EvE Online. We have just over 25,000 people, and well over 100,000 characters in the Alliance. In fact, AFAIK, all of the major alliances have to use Mumble because it allows more than 100 people in a room
Don't you have to host Mumble somewhere? With Discord anyone can create a server and invite friends for free with no technical knowledge required. That's a huge plus. I also remember RadCall was a thing for a while, at least where I am from.
Yes, you need to run the service somewhere. But anyone can do so (Foss).
With discord my experience is limited but I currently understand it's a service model so you're dependant on a company, which can pull the same sh*t teamspeak did at any time.
Not needing any technical knowledge just means someone else is running it, possibly being able to lock you in. And in the case of discord, you already are locked in and have to accept whatever they think up.
I don't think that people are trying to find an eternal solution. Nothing lasts forever. When Discord turns to shit, something else will take its place. There is no need to worry about that when it offers too many advantages today.