The point about the new lens mount is due to the fact that the sensors, as you have hypothesized and drawn them, require a larger image circle than current lenses have.
I fully encourage and enjoy friendly discussions, but also feel actual facts are a sound basis for them even when it is just a thought experiment.
In this diagram of yours the black rectangle represents current 24mm x 36mm sensors, the circle is very close to the image circles projected by RF lenses, ergo, to cover a square sensor the size of your suggested square, you would need new lenses which would require another mount.
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You also ignore the fact that the square format has been around for a long time, indeed I own a 6x6 camera back, and it is an incredibly niche and unpopular aspect ratio, especially now record and CD covers are a severely diminished market.
So please, let’s have a friendly discussion, but let’s include history and physics in it.