Aside from the eye issues, with a phone size screen mounted externally, moving your head 2 in would be like moving your head 2ft, so you couldn't have any varia

Aside from the eye issues, with a phone size screen mounted externally, moving your head 2 in would be like moving your head 2ft, so you couldn't have any variation in your posture without the screen being uncomfortably off-centered. Think of a movie theater where you can move several seats or even rows from the center sweetspot seat and still have roughly the same view.