Isn't that largely what the Warlock was trying to be?
Maybe the original 3.5 Warlock was trying to be a 'simple' caster, to a degree, but mainly it was an at-will caster, it was still more complicated than even a 3.x fighter (though that wasn't the simplest class in 3.x, either, that'd probably have been the Barbarian).
The closest D&D has ever come to a 'simple caster' was probably the HotEC Elemental Sorcerer.
Fighters class gets features that let them contribute in the combat pillar, but nothing for the exploration/social pillars. Barbarian is in a similar boat, although Danger Sense and the strength bonus/damage resistance from Range gives some limited exploration utility.
The Barbarian's every bit as bad off - Raging for a minor OOC advantage is at least as bad as Action Surging to save six seconds in some OOC task.
No other class's abilities revolves so entirely around one pillar, and they all get features that explicitly allow them to contribute in all if not at first level, then very shortly therafter. The Fighter gets nothing for the first couple levels to contribute in anything but its single pillar. The unique extra option for a feat doesn't come up until 6th level.
And it's not unique, everyone gets a feat at 4th, it's just doubling up on that, so the fighter gets the second-best feat two levels early...
All the other fighter types can fight. They bring their "A" game to combat. Is anyone really going to argue that a Paladin or a Ranger or a Barbarian sucks at combat? No, of course not. They all contribute and contribute pretty darn well. But, as soon as we start talking or searching, the ranger gets tracking and spells, the paladin gets Detection and spells, the Barbarian gets FLIGHT FFS.
I think we need to exclude magic that occurs in only one sub-class. The Berserker's every bit as hosed as the Champion and BM.
What does the fighter bring to social or exploration pillar that no one else can do?
That /no one else/ can do? Technically, nothing. But, then, that's technically true of most classes. The Rogue's Expertise, for instance, is not unique, a Bard sub-class also gets it.
Now, there's very little the fighter brings to the other two pillars that /absolutely every other class/ can't also bring. Feats, for instance, anyone can take 'em. Backgrounds, everyone has one. Second Wind? Primarily a combat ability, but lots of classes can heal themselves (and others). Action Surge? Primarily a combat ability, but out of combat can be used to take an extra run action in a sprint or something... the Rogue can get to take an extra action every round, and then there's haste...
The fighter really does define a sort of baseline for the game, what the fighter can do, everyone can do, what each of them do better or worse or in addition that defines the other classes.
It's not putting out as much raw reliable damage as the fighter, but to suggest that a Rogue doesn't contribute anything in a fight is just not true.
The point isn't contributing /anything/. A bog-standard Kobold can contribute to any party, at any level. It can make an untrained skill check or toss a dagger or something, and might roll well. All you need to contribute in any pillar under BA is a warm body - heck, with reptiles and undead and whatnot, it needn't even be warm. That bar is laying on the ground.
The rogue's SA and other combat-applicable features let it contribute significantly more, in combat, than some warm-body with a knife - even a warm body with the same, level, DEX & proficiency. That's pretty meaningful, even if it's not up to the DPR of a Fighter, Barbarian, Paladin or Warlock, nor the durability of a Fighter, Barbarian, Paladin, or Druid.
Out of combat situations are not broken up into discrete rounds, so an extra action isn't applicable there.
It could be in a very specific time-contested task, like a foot race, or raising a portcullis and reaching a lever before the princess is sawed in half or something or something...
I will admit, I was wrong regarding Remarkable Athlete/Student of War/Eldritch Knight spells. They get at least 1 or 2 non-combat pillar option at level 3. And that you're absolutely right in that I don't feel it's very good when by 3rd level, pretty much every other class has gotten a couple of multi-pillar options at 1st, 2nd, and 3rd level.
Keep in mind that the EK's spells are limited to Evocation & Abjuration, at first...
Something at level 1 to help Fighters engage in the other pillars in the same way other classes get to would be heavily appreciated, however.
That'd mostly be in the area of versatility. The fighter is too generic to suggest an OOC specialty like the Ranger's general woodsiness, so it'd have to be some alternate-use sort of thing. Expending an Action Surge or Second Wind for some out-of-combat benefit, for instance.
No. you are arguing semantics without addressing the underlying criticism. Anytime you pick at an all or every type word and have your post revolve around disproving the all or every then you are likely missing the point
It's an internet reflex, like a Newtonian equal-and-opposite-reaction. We're shouting into a pretty big not-quite-void, here, so any absolute statement is going to fall on at least one forumite who can come up with at least one counter-example. And the temptation to be absolutely right in pedantically correcting some other random anonymous poster on the internet seems dreadfully hard to resist. It's really more surprising that you only get one or few such responses, rather than dozens - I guess the thrill wears thin quickly once you have some company...
Even if you're careful to load every post with weasel words, you're likely to get that kind of gleeful 'disproof' now and then.
Ultimately, I guess it says more about the medium than the points being raised.