I think someone will make an app that overrides the IMU measurements so the phone thinks it's in landscape when it's portrait, then use another app to rotate the video to be vertical.
I agree! I wonder if there's already camera apps that do this?
In any case, unless it's in the default camera app and a default option, it will likely do nothing to reduce the plague of vertical video.
I would guess that most people filming something that would be better in landscape didn't even think about it, so won't think about turning an option on.
I realise sensors come in other aspect ratios, but I didn't want to spend the time researching and listing them all. Some sensors are 4:3 (like the IMX363).
But that's irrelevant to my point that the sensor is not square which means you lose more resolution cropping to 16:9 in one orientation (usually portrait) than the other.
I think someone will make an app that overrides the IMU measurements so the phone thinks it's in landscape when it's portrait, then use another app to rotate the video to be vertical.