3catcircus
Adventurer
I was assuming the party was closer to level 9. In any case, it does a good job of highlighting the disparity between classes that can hit hard (like the wizard, rogue, and paladin) vs the classes that can hit more often (such as the fighter, ranger, and monk); in that the latter are incapable of contributing under this model.
Remember, the damage from a weapon attack does not scale at all with level. Only sneak attack and spell slots have scaling damage. It doesn't matter if you're a level 20 fighter, making 4-8 attacks in a round, because each hit still does exactly as much as it was doing at level 1.
Of course, if you did have a weapon that was theoretically capable of killing a dragon, then it would be dead in two hits. It's either invincible, or trivial, with nothing in between.
This is where the Gygaxian multiple attacks but only one hits with any real damage per 6 second round model comes into play - multiple successful attacks per round in a normal hit point based system all add up and subtract from your hp total. Here, since we are comparing to thresholds rather than subtracting, the total damage per round is compared to hit point wound thresholds.
So - a 9th level fighter (assume STR 18 and longsword) that takes an action surge can get off 3 attacks in a round. Best case, he'll do 36 points of damage in a single round - enough to cause moderate wounds - which then combined with an additional moderate wound results in a serious wound.
This easily narrates as the fighters wear down the dragon while waiting for the rogue to get into position for the killing backstab.
I suppose you could, if you wanted to follow the model of subtracting damage from hp, you could alternately leave the existing hit point model and declare wound levels as you get to certain hit point levels (e.g. once you get down to 75% of your total hp, you have a moderate wound), but this doesn't really have the same impact since there is no way to have multiple wounds turn into a more serious injury.