Why not a 21.6mm x 21.6mm sensor?

Great Bustard wrote:

The entire image circle for mFT lenses could fit within such a sensor. While such a sensor would be 25% larger than an APS-C sensor (468 mm² vs 372 mm²), it would still be barely more than half the size of a FF sensor, so I don't imagine the cost of the sensor itself would be an issue.

The advantages of such a sensor, however, are substantial: you would not need to rotate the camera from landscape to portrait orientation (especially convenient if using on-camera bounced flash) and you could crop to any aspect ratio you like with no IQ penalty.

So, what are the downsides that negate the upsides?