There is a huge difference between taking over for a student air traffic controller and a flight student on short final. Yes you should be very confident with a student before you let them get to that point, but they need to eventually get there and you can't know how they will react in that split second before landing. I have never had a student crash with direct supervision, but there have been many times that had the same input been given at a slightly lower altitude and it would have ended badly. We are human and we need to do as much as we can to midigate the risks but every time you fly you accept some risk, both instructor and student.