It seems to me that it's a bit more nuanced than that and likely that Apple has more smarts in there than you describe. They have on-head/off-head detection an

It seems to me that it's a bit more nuanced than that and likely that Apple has more smarts in there than you describe. They have on-head/off-head detection and pause playback when you take them off, so that immediately reduces power draw. Presumably they also have an accelerometer and can go to sleep when they don't move for a few minutes. The case induces a deeper sleep mode that preserves more power, turning off bluetooth, and the processor that's monitoring movement detection.