Steve, the automatic system I mentioned above would immediately descend an equipped airplane (not this accident airplane) to 15,000’ within 15 minutes from 40,000’ (even quicker if a pilot is able to extend speed brakes/spoilers during the episode). So, responsiveness after prolonged exposure would not be an issue in that case.
Perhaps somebody else could speak to the effects of long-term hypoxic exposure, but I imagine that there is a “point of no return.”