... corner sharpness doesn't improve when stopping down like regular lenses do because the corners are streched out and digitally filled with fake pixels to make up what the optics never captured in the first place!
The uncorrected geometric distortion of the RF 14-35 is similar to that of the RF 16. Yet, geometric distortion correction of the RF 14-35 yields very good corner sharpness. That would not be the case if geometric correction inherently introduced significant softness to an image. As I said before, if you really want to understand why the RF 16 is soft in the corners, look somewhere besides geometric distortion correction.
My own observations of pics I've taken with the two lenses show that geometric correction isn't highly destructive to image sharpness. And while the corrected corners of the RF 16 are softer than the RF 14-35, they are still quite acceptable for a majority of uses. It's not like corrected corners of the RF14-35 are merely adequate, and those of the RF 16 are hideous. Far from it.