To be fair, Mumble is FOSS, and Ventrilo and Teamspeak have literally not iterated since 2005. Discord is pretty mediocre software (remember when they accidentally allowed iframe XSS RCE attacks? A very amateurish mistake), but the incumbents were an absolute dumpster fire.
For Mumble in particular, the devs had their heads in the clouds for so long that it is no surprise that it is no longer relevant.
If you had a mic that had issues in any way (buzzing, volume, balance), "The Wizard" and "AGC" were supposed to fix it for you. Do not fret little one, for you do not need nor want to manually fiddle with settings, The Wizard will make everything right [1]!
The pivotal feature that was the reason so many people I know stopped using it is the ability to change the volume of an individual person [2]. It has been a requested feature since the beginning of time, yet it took until 2016 to implement in dev branch and didn't actually make it into a release version until 2020! Too little, too late.
That issue is pretty amazing, especially the developer straight-up accusing those saying 'this is a killer feature stopping us from moving to mumble' of lying (considering the substantial overlap between the features of the various options competing in the space at the time this is really obviously the kind of thing which could be a deal-breaker: I remember the arguments over which option to use based on far less substantial differences than this).
To be fair, Mumble is FOSS, and Ventrilo and Teamspeak have literally not iterated since 2005. Discord is pretty mediocre software (remember when they accidentally allowed iframe XSS RCE attacks? A very amateurish mistake), but the incumbents were an absolute dumpster fire.