Your eyes are not the most accurate. From 20,000' are you telling me you can pinpoint the precise peice of ground you are over, accurate to probably 10m, but certainly 100m?
GPS monitoring is far better than "simple VOR or ILS"....let alone NDB. I've seen a perfectly servicable ILS, which I was flying down on autopilot distort due to another aircraft on the ground. This sent the autopilot chasing the needles, and induced a pretty strange oscillation in close proximity to the ground. No warnings at all.
Anyway, soon we'll be able to fly the "virtual" glideslope in most of the world....other than the UK. Probably because old ex-RAF navigators sit on the boards that make the decisions. You're not one of those DFC are you?