Sure, that’s a valid reason for a broken use case … but it’s still a broken use case. If your mics weren’t broken then the other modes would be better.
Agreed, but quality products are designed to continue operating as best as possible when components fail. You can definitely take that overboard to extreme level, but a basic think through of especially common failure causes (like ear junk buildup) and design around it is what great product designers do.
Based on the noises I experienced that time, I think the microphone was detached and hanging by a wire. The sound was rhythmic and depended on my movement, kind of like what would happen if you had a bell hanging on a pendulum inside a cylinder. I don’t think this was caused by debris buildup on the device. To Apple’s credit, whenever I had a defective AirPod, it was easy to get them to replace it. Happened to me multiple times in the first few months of the AirPods Pro release.