That is not a penalty. It is not even a concrete state. It is nothing but an abstract game-mechanical notation.Being down 5 HP is an imposed combat penalty.
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being down 5 HP means being 5 HP worse at fighting and 5 HP more likely to lose the fight rather than win because of the effects of the wounds.
In the real world, there is no such thing as "being 5 hp more likely to lose the fight". There is such a thing as being winded, or tired, or blinded by sweat/blood. All these things can make you more likely to lose a fight. There is also such a thing as being out-matched, or wrong-footed, or feinted. All these things can make you more likely to lose a fight, too. But none of these things, in either list, takes 5 days to recover from. Either you lose the fight because of it; or you nevertheless win the fight and then get your breath back, wipe your brow, sheath your sword.
If hit point loss represents situational disadvantages of the sort I've described, then the 4e/5e approach makes more sense. If it represents serious, debilitating physical injury, then the earlier edition approach fails to model it very well, due to the lack of penalties and the overly-rapid healing times (hence the reason why games that do model such things, eg RQ, RM, GURPS, were invented and built up big followings in the 80s and early 90s).
Being wounded does not "magically" slow a person down. It slows them down by way of physical - indeed, physiological - mechanisms. A twisted or sprained ankle makes it harder to run. A broken toe can make it harder to walk or even stand. An injured shoulder can limit the extent of arm movements.It is just bizarre people keep insisting that being wounded magically slows you down or makes it harder to hit someone. This has no basis in reality Yes certain major injuries to specific body parts may impose a specific penalty, but that would involve tracking hit locations, damage per location, having tables to randomize the injury etc.
An injury that is not serious enough to impede physical functioning - particular movement - is not going to kill anyone. And is not going to impede anyone's combat performance. A scratch to my arm won't slow me down, but it won't make me lose a fight, either. So hit point loss, which does track a progression towards losing a fight, can't simply be this sort of thing either.
Another reason it can't be this sort of things would be that high level AD&D fighters take a day to heal a scratch that a low-level character doesn't even notice - which is silly.
Hit point loss therefore has to correlate to a trajectory towards defeat, but without being seriously injured. Hence Gygax's express emphasis on luck and divine fortune, or the implicit 4e emphasis on verve, morale and energy (implicit in the name of the healing mechanics, like Healing Surge, Second Wind, Inspiring Word, etc).
Gygax doesn't say that every hit point has a meat component.What he made clear was every HP has a meat component to it because every mechanic in the game that reduces it requires it, from poison to additional weapon effects to losing HPs from attacks you don't even know were there.
The whole rationale is that it is flexible. You can narrate it this way on this occasion, and this other way on this other occasion. Even losing hit points to an attack you didn't know was there doesn't need to be narrated as "meat loss": if a 10th level thief backstabs a 10th level fighter for 40 hp of damage, and the fighter has 40 hp remaining, there is no obligation to narrate that as meat - drawing on Gygax's idea of hit points as "sixth sense", you can run with REH-esque narration of a "pantherish twist" by the fighter at the last instance, as s/he feels the rush of air from the descending blade and turns aside, taking only a nick or scrape or being left off-guard and vulnerable to a follow-up attack.
Even if the thief's blade is poisoned, there is no need to narrate "meat" loss if the fighter makes his/her poison save (as Gygax expressly calls out in the discussion of saving throws in his DMG).
If the dart hits, and the poison takes effect, then actual physical contact must be narrated - but not necessarily "meat" loss (a blowgun dart doesn't do any genuine meat damage unless it hits you in the eye). But if the poison save is made, then there is no need to narrate any physical contact at all - "You narrowly dodge the blown dart - lose 1 hp" is a completely feasible narration.1 HP damage injection poison darts REQUIRE it.