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The notch helps in securing the case."The notch" usually refers to the screen cutout for sensors at the top of the iPhone X screen, which has no relevance to the case. Do you mean the camera bump?
The iPhone cases from Apple would grip the same without the camera bump and corresponding case cutout. The sides of the apple cases wrap around all four edges of the phone, with only a partial cutout for speakers, microphone, and lightning port at the bottom. The edges of the case hold it very securely. It's hard to get the phone out because the edges grip so tightly.
> The changing shape of the notch also means that cases don't transfer between iPhone models.
The rest of the dimensions also change. If the iPhone 7 and 8 had the camera in the exact same spot, the cases still wouldn't be interchangeable. Not with a good fit on each phone at least.
> I guarantee Jony uses a case.
I bet he doesn't. People use cases to protect their phones. Ive could break a hundred phones and not feel any pain from replacing them.