There are a number of assumptions missing in this. Emergency approaches are by their very nature often well below glide slope. Perhaps more statistically rele

There are a number of assumptions missing in this. Emergency approaches are by their very nature often well below glide slope. Perhaps more statistically relevant is that not everyone's altimeters always function correctly, are set to the correct airfield's most recent numbers, that in the potentially 50+ minutes since the last ATIS update the ambient pressure hasn't changed significantly, and lastly that all pilots–drone pilots included–are doing what they're supposed to and leaving sufficient margins. Shoot enough approaches and you will likely realize that small, potentially dangerous objects piloted by untrained, unregulated, unmonitored, non-communicating pilots at 400' could quickly become a problem for you more readily than it appears from the ground. Take off on a high-density-altitude day in an underpowered aircraft and you'll also appreciate the additional headroom. To say nothing of the fact that I've owned drones that under LoS conditions just continue the last input received, which with inexperienced or irresponsible operators would be an input to climb.