> The technical reason is simple: Bluetooth has a low bandwidth
Nope. The reason is that to listen, Bluetooth uses the unidirectional A2DP profile, while the only two-way profiles are HSP and HFP from 20 years ago, which are monaural and favor low latency over quality, with an old codec and a passband filter focusing on human voice.
Forcing the mic to be anything but the BT one makes the stack move to A2DP (with infinitely better listening quality for you at the cost of a teeny bit of latency) and, well, a local
mic, possibly even an array if mics in the MacBook case (with infinitely better quality for the other side + ~zero latency)
If both ends do the no-BT-mic dance the quality is crystal clear + BT latency is only paid once (twice RTT) instead of twice (four times RTT)
On macOS one can option-click on the Volume menubar item and switch audio input in a pinch.
https://github.com/Gaulomatic/AirPodsSanity
I didn't create this, but I do detest subscription models on bugfixes..