>Hypothesis: A growing number of cellular subscribers don't care enough or use voice calls as much, so (regardless of motive/feelings) cellular providers have found they can simply reduce quality without losing customers.

This makes absolutely zero sense because calls take negligible bandwidth, especially with VoLTE. Not to mention the whole premise isn't even internally consistent. "People don't use it enough, so let's try to save money by making them use it even less! But only randomly and at the margins, so we still have to keep everything running!"