I don't understand why you keep say "as per the OP" for things that I can't find in the OP.
Here:
So, I started thinking about the title while reading a Reddit thread about adventurers with disabilities, many people there were claiming it was a ridiculous concept, since they couldn't possibly survive the dangers in dungeons and such. I initially thought it was silly argument, since in the games I've played even a completely healthy person would probably die a hundred times over between all the fights with giant monsters, being hit by lightning, fire, explosions, bullets, and waves of poison, getting beaten to a pulp and completely recovering in a night, etc. The PCs only survive due to superhuman abilities granted by their classes and general D&D mechanics, all of which a disabled character would have as well.
But that's not necessarily how people play, and a lot of games have more grounded situations for the players to face, so I figured I could ask here what people think. Could real people survive in the games you've played or DM'd? A party with normal human limits, with no class abilities, magic, luck, divine favor or any game-granted ability that wouldn't be possible in real life. They can't use any modern technology or metagame knowledge, but they can be as capable as a real person could physically be (so 200 IQ Navy Seals are fine), and any magic items a regular party would find along the way are fair game as well.
Could they complete your adventurers, or at least survive all the challenges? If not, where do you think they'd fail?
The "no class abilities" IMO basically means no class, because every feature provided by a class is a class ability. That include hit points, proficiencies, etc. Including to magic, etc. as well.
Being a "real human" doesn't mean you can't have levels. In fact, 3e had rules for high level mundane NPCs that explicitly had no special abilities, but still had things like HP, skills, and other basic class mechanics. So you could easily have a level 20 Commoner or Aristocrat.
You'd have to ask
@Zubatcarteira concerning that. The bolded part above would indicate those rules wouldn't be applicable IMO, so I go with that.
That's how you fight superior foe. Ambush tactics. Not so remote, it was part of dwarven kingdom, one of their supply routes. Dragon wasn't stupid. It knew dwarfs use those mountain roads to haul goods, which was primo place for easy fly by plundering of livestock and food supply. So party tracked dragon movements, patterns, with help from caravan people and route wardens. Extensive scouting and research about foe was done, so was in choosing right time and spot for setting ambush point.
Yeah, it was sort of stupid if it acted like that. But hey, white dragons were the lowest intelligence dragon, so sure...
But a 13 HD, 60-ish HP, AC -1
Lv 4 in 2ed AD&D. Our party fighter had 15HP, ranger 18, thief had 10, wizard was at 8. Don't know if you played it, but you rolled for HP at every level, even first. So fighter with 1 hp at lv1 was also possible. Also, stats were 3d6, arrange as you wish, so good luck getting that extra 15 needed to put into CON for extra 1 hp per level. Normal human would definitely fall into lv 3-4 ad&d character. As in, 1-2 good hits from sword and you are dead.
Yeah, I know how AD&D played, 1E as well as 2E. Played it for decades.
Fighter with 1 hp at 1st level? Wouldn't have made it to 4th IMO. Stats were 3d6 arrange in 2E, but they were 4d6 drop lowest in 1E, method V for 2E IIRC. I'll post the pages if you don't remember that.
I really don't know anyone who played 2E with 3d6, arranged. Most came from 1E, where 4d6 drop lowest was the norm and default method. Only those from B/X embraced 3d6, where bonuses began at 12. Anyway, most groups rolled several times to get a PC with decent scores, regardless of method. IME usually a minimum of 3 sets of scores.
However, all that is neither here nor there...
A "normal human" is not a 3-4 level AD&D character. You should also know "normal humans" rolled 3d6, treating 1's as 3's and 6's as 4's according to the DMG 1E. I'd guess 2E was the same, but would have to dig through my books to verify that and it might not be included in 2E.
Also Fighters had weapon specialization, Rangers giant-class foes, Magic-Users / Mages spells... all things "normal humans" don't have. Again, refer to the OP for what qualifies for this discussion/thread as "normal human".
So, no, normal humans were not really close to adventurers.
No. It was just few sessions of extensive prep work to set things in place, formulate plan, gather equipment, resources etc. Overlapping fields of fire, negating one big dragons advantage (flight), surprise attack with artillery, and of course, right spot. Pillboxes where just basic round low stone buildings with slits for crossbows, same type farmers build across our islands for hundreds of years ( they are built across Mediterranean region,specially islands) as shelter from storms. Built couple with my grandpa as a kid in our olive gardens. Fighting dragon in narrow gorge is like infantry fighting tanks in urban environment, same tactical principles, modified for fantasy.
FWIW, I know about pill boxes, military strategy, etc. I don't need the details on that, myself.
I was more talking about some of the resources you had available: steel cables for example? Not something you'd find in most D&D games IMO. Not to mention getting everything in position without the dragon learning about it? If it watched the trade route for prey, etc. seems a bit of a stretch it would never have noticed the movement of scorpios into positions, attaching steel cabels to the mountains, building the pillboxes, and every thing else you did for prep.
As i said, 2ed was much more realistic, specially low levels. Low hp, low to hit, you needed way higher stats for bonuses (at least 15, sometimes even 16). So normal humans have same chance as PCs to survive low level adventures. They just have to play it very smart and don't try to act a hero.
More realistic, yes, but not realistic when it comes to "normal humans". Commoners in 5E, by comparison, have a chance of survival not much worse than some 1st-level characters in 5E, if they "play it very smart and don't try to act a hero."
If you run your game that way, then sure, a normal human might make it as an adventurer, but IME no one plays their games that way.