The thinnest smartphone ever sold was the Vivo X5 Max, which was 4.75 mm thick, and it still managed to squeeze a headphone jack into it, despite Apple's protests that such things are impossible. USB-C (2.5 mm) is significantly thinner than a standard headphone jack (3.5 mm). Between the screen, the circuit board, the battery, and the back cover (including the camera lenses), you're never, ever going to see a smartphone too thin to fit a 2.5 mm port. This is a non-excuse.