Good post, SeekerRuth. A response like you suggest (that everyone has a parachute under his or her seat, they just choose not to believe the plane is going down) is a red herring. I think the implicit argument in the parable still stands without adding any new elements.
The story aims to put two alternatives about the afterlife side by side and to determine which is more good. In the first scenario, there is no afterlife; oblivion awaits everyone after death. The second is that the vast majority of people are going to be eternally tormented and a very few will have eternal bliss. I would aver that oblivion for all is better than pleasure for a very few and eternal torment for the vast majority.
If an interlocutor of mine replied by saying "Well, everyone could go to heaven, they just choose not to; it's their own fault," I would respond by making him or her defend the proposition that most people in hell + a few in heaven > no people at all. The defense of that ground will lead them into absurdities. Unfortunately, most people have a pretty high tolerance for those, when they're on the producing end.
But let us argue by analogy again. Let us suppose a similar set up to the above story but this time everyone knows and believes that the plane is going to crash and, since no one has a death wish, all want to escape the doomed aircraft. Happily, a parachute is stored under each seat on the plane. Unfortunately, the directions, being written in Greek, are very difficult for most passengers to understand and, due to the small amount of time before the plane hits the mountain and the great complexity of the devices, those few who do manage to get their parachutes on properly have no time to assist those who do not.
Ask your interlocutor if the airline acted responsibly if they knew that (1) studies indicated that 90% of passengers wouldn't be able to use their parachutes to escape in an emergency and (2) the plane had a very high (indeed, 100%) chance of crashing. I think that if you condemn the airline for their actions you'd also need to condemn God for making a universe which will result in much more suffering than good.