The O-ring that failed was never part of the Challenger shuttle. It was in one of the solid rocket boosters that was supposed to aid in the shuttle's ascent. It's ironic that he's trying to argue for something to be accurate and he can't even get his facts straight, but then again, David has never cared much for facts.
Furthermore, something in a rocket can fail and the mission can still be successful. Modern rocket engines are designed with backup sensors, valves, pumps, cooling systems, etc. when applicable to help deal with primary system failures. This makes his analogy quite terrible. There have also been many instances in space exploration where important systems have failed and there was still huge success to be had:
Apollo 13 had a major oxygen leak and the crew still managed to fly around the Moon and come home after jury rigging an adapter for a CO2 scrubber.
Gravity Probe B's gyroscopes were not working properly and this required some serious error analysis to be preformed on its data. The probe verified the existence of geodetic effect and frame-dragging as predicted by the theory of general relativity.
Galileo was still able to transmit data after its high-gain antenna failed to open correctly.
Skylab's micrometeoroid shield detached and tore away a solar array, made it so another array couldn't open, and took away the thermal shield which was meant to reduce solar heating. The first crew aboard the station had to manually unfold the remaining solar array and put a reflective shroud over the station to prevent overheating.
We all remember how Hubble was saved when it got its corrective lenses.
Kepler had a reaction wheel failure four years after its mission started. The failure was accommodated for by using solar radiation pressure to help control the crafts attitude and maintain focus. Although Kepler lost track of its intended target it was able to successfully gather data in other parts of the sky.
Spirit had a wheel failure and engineers worked around it by driving the rover backwards.
And those are just the ones I know of, there are definitely many more.