I think the biggest hurdle is lack of precision. 200 feet isn't much error in the horizontal realm but is a big deal vertically. There are ways to supplement t

I think the biggest hurdle is lack of precision. 200 feet isn't much error in the horizontal realm but is a big deal vertically. There are ways to supplement the precision of GPS altitude with ground-based stations (WAAS), and this is indeed used to construct GPS-based instrument approach procedures which are comparable in accuracy to traditional means (ILS). But these require ground-based transmitters; impractical to implement worldwide.