I don't understand why you think that? You object to a farm boy being a fighter because they haven't got training, but you also object to the farm boy being a fighter because they have special powers because they are the son of a god/Darth Vader/the rightful king/the Chosen One which stands instead of training?
The problem for 5E is that an L1 Fighter is an extremely competent and well-trained combatant, who knows every kind of armour and every normal weapon out there, and has fighting so developed, he has a "style". I can definitely see Second Wind and Action Surge as just "heroic" qualities sure.
But that's already not a "farm boy", because of that level of training.
They don't get any special powers, if they have them, until level 3, and that's kind of problematic. If they got them at L1 it would much easier to say "Okay this dude has these powers because of the Force" or whatever. Chosen One-types frequently do need and have had a lot of training. Rand al'Thor comes to mind. He'd been trained for a long time by his dad, and you don't get much more "chosen one" than him.
This is a situation created by the rules conflicting with what you want to do with them. 5E doesn't have a great "Chosen One" class. Monk actually might be the best. You can re-skin Monk very easily (just as we'd want to re-skin Fighter to be a "farm boy", but all the proficiencies etc. make it weird). Indeed his unarmed attack, lack of need for armour, and ability to make a wide variety of weapons deadly actually scream "Chosen One" from fantasy.
I think it's fair to say that Fighter doesn't necessarily reflect a soldier, from an army, but it does reflect someone who has been trained to fight. I guess you could say "I was never trained, I just instinctively know how to do these things! It's a mystery!". That's cool, but it's like, definitely the default assumption.
Those modern fantasy novels are perfectly good, many are much better written than the pulp fantasies that influenced Gygax, but they are not representative of the whole of the genre.
I disagree. Genres are not stuck in amber in the 1970s. They progress and change. Writers are read and writers are forgotten. Tolkien is NOT the main touchstone for D&D, the pulp that Gygax read is. Gygax and others have been very clear about this. The Tolkien stuff was almost against Gygax's will, to hear him (and others) tell it. Early D&D's structure and goals are those, primarily, of sword and sorcery. So all this about "Tolkien wrote as he wanted the world to be!" is fine, but RE Howard and Moorcock and so on, all as big or bigger influences on D&D did not. Their stuff was every bit as dark and gritty as, say GRRM or Joe Abercrombie. The genre as a whole is far closer to Moorcock, now, than Tolkien, and so is D&D, I would suggest.