Here is what I think:
*In the original depiction it was simple:
Fighter= The guy with sword
Wizard= The guy with wand
Most of the visual aesthetic was Arthurian...
Old wizards in pointy hats + armoured knights etc.
*And in time, as more books came, other influences began to stick and stack to the core concept of this game of sword and magic...
-With the re-discovery of Conan in 70s, came the Barbarian.
-With so much love for Aragorn & LotR, came the Ranger.
_Due to some academic books on the subject, secret orders of assassins (the Hashishi cult) were popular...so the Assassin class was integrated.
_with efforts to spice-up and vary the Wizard class, they got the Druid.
-And then, with the 90s & 2000s, the inevitable influences like "badass anti-heroes" + "manga" creeped in, and D&D had to adapt....thus they created cool archetypes like Dragonborn Warlord etc.
(which are not inferior to the previous archetypes, but have no precedents in any mythological and/or literary text)
*So we got to 2012...where we can't find a proper meaning and context for the Fighter. All because of the many class inflation.
But it's actually the clearest of the classes to understand, Fighter is that guy with the sword , fighting the monster at the front line.
-He's a axe-hurling Dane in a Beowulf campaign
-He is Lancelot the Fighter betraying Arthur the Paladin or searching the grail with Galahad the Cleric in an Arthurian campaign.
-He is one of Conan's Hyrkanian war-dogs in a Hyborian Age campaign
-He is an Argonaut, or one of the Troyans in a mythical Greece campaign,
_He is a Gondorrian foot soldier or a Rohirrim rider in a Middle-earth campaign.
etc.
He is Lİttle John fighting with Robin Hood the Ranger in a Sherwood campaign
_ A final metaphor:
D&D is an open buffet with rich choices everywhere...but imagine a person eating all the salads, soups, steaks and desserts at the same time...then he gets sick, unable to appreciate the unique exquisite taste of a specific food.
D&D is also the same, it is an open feast of myth filled with men carrying swords and wands...but trying to fill them all in the same game dims their individual tastes.
I think we should pick & choose...
i.e.
There are no knights-paladins in classic Howardian sword & sorcery, and almost no good wizards.
On the other hand, there are no heroic barbarians in Arthurian myths
Just because D&D brought them side by side
doesn't mean we should have Arthur, Gandalf and Conan, fighting Dracula
The Bottom Line:
D&D is a conglomeration of wonderful mythologies...which deserve to be focused on separately, instead of getting lost in eclectic mish-mash of "generic settings"