The classic Hero's trope of farmboy to legend is not represented well by the D&D Fighter, and I would argue that has always been true, at least as long as I've been playing (2e AD&D start).
I would agree 100%.
Farmboy hero is a 0th level dude, and not really in D&D.
Lets look at a few examples:
(admittedly not a fighter) Bilbo Baggins in The Hobbit. 1st level rogue at the start of the story. He has no training in that department - he is basically one of the idle rich - but Gandalf spots an inherent aptitude. Later on in the story, after gaining a few levels, he is battling it out solo against giant spiders (who already took out all the trained fighters). There is no training montage, no mention of anyone teaching him which end to hold a sword. Gandalf is his mentor, but he doesn't teach any specific rogue skills.
Another example. Aliens. We have a ship full of highly trained professional soldiers, who get slaughtered, half of them in the space of few minutes. Who is it that goes in and defeats the enemy boss, duel wielding military grade weaponry? Ellen Ripley, a junior officer on a cargo ship with no military training.
In the universe of stories military experience don't mean jack.
And I can find dozens of examples where military experience is matters. You claim Bilbo is a "Rogue", but that's just a rubbish approximation. He possesses a fraction of the skills of the Rogue/Thief, seems to be missing Backstab, and his ability to fight with a sword is a PLOT HOLE, not something to be praised. A D&D Thief wasn't intended to be Bilbo Baggins, he was intended to The Grey Mouser, who does train, and practice with his weapons (except when he got old and fat and into teenage girls, but that was implied to be a bad thing).
Aliens is a very different kind of story to D&D, and D&D is completely incapable of replicating something like that, because a class-based system with a heavy reliance on proficiency is hard-incompatible with the story there, unless maybe Ripley was, like, way higher level than everyone else. But plenty of other RPGs would be fine with it. GURPS or Savage Worlds or even Cyberpunk 2020 would all handle it a hell of a lot better. So that doesn't really help your point.
And in the vast majority of fantasy novels, military training and combat experience is important, whether it's the Assassin trilogy by Robin Hobb, Game of Thrones by GRRM, Mary Gentle's Ash series, Glen Cook's Black Company series, arguably Stevenson's Malazan (but that is all over the place on this, at times it matters, at times magic means it doesn't), Joe Abercrombie's First Law-world books, or whatever. Military training isn't everything - often magic, or trickery the like beats it, but it's very important and the best fighters in these books are usually well-trained and/or extreme veterans. They sure aren't "farm boys".
What's particularly hilarious is that the classic "farm boy" everyone is obsessed with wouldn't even be a Fighter in D&D, he'd be a Psychic Warrior if he was a D&D class at all, because he had MASSIVE MAGICAL POWERS (Luke Skywalker that is), which are what made him so capable.