Just looking at the tech specs for the iPhone 11 (lightning) [1] versus iPad Pro (USB-C) [2], the former is actually thicker at 8.3 mm compared to 5.9 mm, and yet the iPad Pro can have USB-C.
Especially given that it's so prevalent in their other hardware, it almost seems like stubbornness at this point that Apple doesn't want to switch, but admittedly that doesn't pass the test of Hanlon's razor. It seems like there must be a better explanation out there somewhere for why they've desperately clung to lightning for so long (backward cable compatibility within the iPhone ecosystem? inertia in circuit/case design?).
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