There are only three ways the drone could know its AGL reading. 1. using Internet and accurate topo maps. 2. Radar. 3. LIDAR. First option would require internet access during flight. Not gonna happen for a lot of folks. Second option ONLY way that could happen is if they started putting ground radar on the copters. The sonar can't possibly reach 400'. Baro is air pressure and has squat to do with AGL. The same baro reading would be valid if you were sitting on a 400' tall hill or 400' up in the air from 0 AMSL. The Baro calculations are a relative, meaning relative from set point, ie the home point I would guess. Ground radar is heavy, expensive and complicated. Not happening on a $1k copter. Last is LIDAR. Possible. However, inaccurate on reflective surfaces (water for example). And I am not sure how heavy that would be and how accurate it could be in a small package.
Would I like it? Sure. I would also like horizontal LIDIR for terrain and obstetrical avoidance during RTH. If it sees something in front of it, fly over it. Sure would be nice. Again, weight vs flight time issue. More stuff, bigger batteries, bigger motors. Its a vicious circle.