Phones already have proximity sensors and facial recognition. We're just not using them to scale pixels yet. But for most intents and purposes, it's enough to

Phones already have proximity sensors and facial recognition. We're just not using them to scale pixels yet. But for most intents and purposes, it's enough to assume a "typical" distance for the type of device in question. People with bad eyesight are free to place the device closer to their eyes, zoom in, or adjust a system setting that makes everything look larger.