Are crippling disabilities and yet more ways to meet instant death desirable in an open-ended, episodic game where participants seek to identify with lovingly detailed and developed player-character personae? Not likely! Certain death is as undesirable as a give-away campaign.
Based on this, one can surmise that the lack of hit locations, critical hits, and instant death aren't meant to be part of the D&D game. Of course, one would be guilty of taking things out of context. I'm sure that one can come up with spells and artifacts that resulted in instant death, blah blah blah.
The point is, there was a concern early on that D&D combat wasn't meant to be 'realistic' but 'plausible' from a certain point of view. It wanted players to have a decent chance of surviving combat (which by no means is an argument for the 'Challenge Rating' approach) with options of running away as part of the combat sequence. And one might argue -- based on this statement -- that despite the fragility of 1st level characters, D&D PCs are meant to overcome challenges and progress in a well-run (and well-played) campaign.