Sure, if you stop after "nobody's vocal coords make noises above 4khz in normal conversation", but the rumbling of the vocal coords isn't the entire audio data

Sure, if you stop after "nobody's vocal coords make noises above 4khz in normal conversation", but the rumbling of the vocal coords isn't the entire audio data which is present in-person. Clicks of the tongue and smacking of the lips make much higher frequencies, and higher sample rates capture the timbre/shape of the soundwave instead of rounding it down to a smooth sine wave. Discord defaults to 64kbps, but you can push it up to 96kbps or 128kbps with nitro membership, and it's not hard to hear an improvement with the higher bitrates. And if you've ever used bluetooth audio, you know the difference in quality between the bidirectional call profile, and the unidirectional music profile, and wished to have the bandwidth of the music profile with the low latency of the call profile.