With all the downsides to sapphire (cost, thickness, light transmission), you start wondering if the previous iPhones were using this material just as a marketing point and not because there was an actual benefit to the users.
They’ve been using it on the camera lenses and the surface of the fingerprint sensor, not on the display.
In both of those applications, scratch resistance is extremely important. Also, those are both small, which makes them cheaper and easier to engineer, as well as less likely to shatter.
