Fanaelialae
Legend
I don't think RAW says much about walking off a cliff. Any more than it does about a character stabbing themselves in the heart, or smoking a stick of dynamite.No they dont. Damage disproportionately impacts classes with less HP than it does classes with more HP.
Deal 10 damage to a 1st level Wizard. Now do it to the 1st level Barbarian.
Higher HD is a fundamental class feature of martials, granting them luck and plot armor typical of the sword using hero of fiction on which DnD is heavily based. If you're ruling 'insta-death' for things that RAW are simply HP damage, then you're depriving martials of the advantages of that class feature.
Even if I were to accept your argument at face value, this would be no more unfair than any number of abilities that ignore hp, such as a banshee's wail which takes you directly to 0 hp. Or a creature like a beholder, which can neutralize magic.
As I see it, HP are intended for non-suicidal actions. Not for sticking your head between a dragon's jaws to see whether your skull is stronger that their bite.
I completely disagree that this deprives martials of anything. It limits the player's ability to meta game, sure. However, it doesn't limit the character, IMO, because I don't for one second believe that the HP and falling rules were designed with the intent of allowing high level characters to walk off a tall cliff as a means of getting more quickly to the botton.