I think it's a matter of taste, but I think there are some differences in viewpoints.
It seems like you're picturing an adventuring party like a SEAL team -- where everyone are highly-trained experts who can do anything and are all self-reliant. But I think AD&D pictured groups more as historical expeditions. They were likely to have a bunch of lesser combatants (henchmen) as well non-combatants like porters, torch-bearers, squires, hirelings, etc. I think of the Lewis and Clark expedition that had a bunch of unmarried soldiers but also boat crew, an trapper/interpreter and his pregnant wife, and an enslaved body servant.
The problem I have with the Lewis and Clark analogy is that D&D adventures in general, and dungeons in particular, are phenomenally dangerous. Much more dangerous than even the American frontier was, and more dangerous than most historical wars. A D&D party goes into a dungeon fully expecting to face deadly traps and be in multiple life-or-death conflicts with supernatural creatures before they return, not to mention the likelihood they'll have to scale sheer surfaces or swim underwater for significant distances. I can't imagine anyone willingly taking a pregnant woman into that.
I don't know about the Navy Seal comparison. Frankly, I don't know that much about Navy Seals, but my understanding is that their job is to be elite combatants first, and everything else is in service to that. Maybe SHARK's "every Marine a rifleman" is a better analogy. I don't think every adventurer needs to be an elite fighter or necessarily an elite anything. As I said in the other thread, it strikes me as a profession which would reward generalism more than specialization. I accept class roles in D&D, because it's D&D and that's just how the game works, but I do think skill-based systems can sometimes produce more plausible, well-rounded adventurers out of character creation.
If we're open to literary examples, I'd point to adventure-fiction characters of the Indiana Jones/Alan Quartermain/Nathan Drake type. They're not usually the absolute best at anything, but they have a broad skill and knowledge base, and above-average competence in the things most important to what they do. Indiana Jones gets his ass whipped all the time, but he can still scrap. In the fantasy realm, characters like Conan or Fafhrd and the Grey Mouser are elite fighters, but they're also generalists who have plenty of skills beyond fighting.
I also question your comment about can't withstand physical hardship. In my experience, most magic users have low Strength but do have high Constitution, and I think of them as being quite tough and able to endure hardship. They're just not very skilled at fighting.
That has not been my experience, but obviously it depends on the luck of the die for attribute scores. I think it's fair to say that every other measure of physical resilience D&D tracks, wizards traditionally score very poorly. They always have the lowest hit die and usually have the worst poison or constitution saves. They usually don't have athletics as a class skill, and they don't get any class features related to travel or survival.
They're always bad at fighting, but the relevant question is how bad? Because they shouldn't be as good as a fighter or thief. What gets my goat is when people say that wizards (and thieves) aren't supposed to be able to fight, and shouldn't be fighting. Anyone who walks into an RPG dungeon should expect to be in a fight. If they're likely to be slaughtered as soon as a 1 hit-die goblin gets into melee with them, they have no business being there.
There are exceptions, I'm sure. I never read any Dragonlance fiction, but I understand that Raistlin was portrayed as sickly - and that may have become a stereotype of D&D, but I'm not sure it was part of the original vision. Gandalf was the earlier stereotype for the wizard, and he was technically an extremely tough demigod who wielded a sword on his horse in battle. AD&D magic users weren't Gandalf, for sure, but they weren't necessarily delicate flowers who couldn't endure hardship.
I don't know if Raistlin is to blame or just the general stereotype that wizards are all egghead nerds. Gandalf is better, but it's a bit unfair because he's a superhuman. There aren't many great literary models for a wizard adventurer, because in most classic fantasy, wizards aren't adventurers. They're villains or advisor-types. Turjan from Dying Earth is probably the most apt example, but Turjan can handle a sword. In fact, he and Cugel are both good examples of why a magic-user in a Vancian system better be able to defend themselves without magic.