In d&d land, there are two kinds of people: Those who have magic, and those who don't.
There a reason I mostly play spellcasters in d&d: I like having agency.
D&D has always been a disfunctional premise on a concept level from the moment it allows Haves and Have Nots in a same group, and if a player at my table insists on playing a muggle then so be it, but I warn said player that they're playing a concept with a limited shelf life.
This is not only as a player. As a GM I don't like dealing with magical Have Nots because they limit the stories I can tell and adventures I can run. D&D character levels go up and eventually "go hit the neighbouring orcs with a pointy stick" must grow into "ask the overdeity's oracle for the archfiend's true name, for which you have to go to a lost demiplane with access point in a sunken city under the sea," something the protagonist will never be able to do if all they can do is hitting things with a pointy stick. If I'm going to run an adventure, the character party must be able to pull their weight on their own power, because if I have to keep solving their every problem via McGuffins and Deux Ex Machina NPCs, then there was never a challenge, the whole thing is a travesty.
Fictional "muggle heroes" can be divided in two:
1) The Vanilla Action Hero: All this guy can do is fight. He's usually a blockhead whose fighting ability is only rivaled by his bull-headedness. Because of his lack of ambition, the call to adventure always falls straight on his lap, and because of his lack of brains or skills, the stumbles through the plot advancing only because of the purest serendipity, everything he needs just falls at his feet via the divine providence, and his foes tend to lose because of some contrived circumstance that creates an lethal opening at the exact point in time.
(The girl fell into his taxi without any input or effort from him: Check
The army literally knocked on his door to give him the means to get into
the resort cruise in space to meet the contact: Check.
The girl gave him every clue short of outright spelling-it-out for him to
figure out what the Fifth Element was: Check)
2) The Badass Normal: In a fantasy world (call it magic, superpowers, etc), the badass normal is a "non-superpowered person"... or rather, someone with only one superpower: Hypercompetence. The BN is ridiculously fit (regardless of apparent muscle mass), unrealistically resourceful, and crazy-prepared; they're all that and an order of fries; the pinnacle of human ambition. While people misleadingly sell BNs as "heroes like you," the thing about BNs in fiction is that, once you actually get to assess they things they do on a regular basis, you realize there's nothing "normal" about said badasses. Plus, they're often impossible to emulate in class-based games... or rather just impossible, period. Just the list of accomplishments of ANY BN (even Hawkeye's!) is impossible to normal humans in and of itself. If anything, being a BN is in itself a superpower.
(World's Greatest Detective... who also happens to be world's top weight lifter,
best martial artist, accomplished gadgeteer, all-round Modern Man of the
Renaissance, billionaire, decked with technology at least ten years ahead of his time,
and has his life sorted out just the right way so his two lives never come in conflict in
any way... sounds legit)
D&D biggest offense regarding the Fighter is that they sell you a Badass Normal but what they give you is a Vanilla Action Hero, and the Vanilla Action Hero works only in literature and otherwise passive media because the author arbitrarily resolves the hero's problems via contrivances, serendipity, and Swords Of Plot Advancement whenever is dramatically appropriate. If this VAH had no Plot Armor and a "benevolent God," he'd have no way to reach the underwater city, let alone traverse between the planes, or defeat a god.
In addition, I don't like my players getting frustrated, and classes like the Fighter are nothing but a recipe for frustration. All (useful) character classes bring something to the table, and ALL classes fight, so if all your character can do is fight then you're not contributing anything to the adventure. Furthermore, eventually the fighter can't do even that (fight) because eventually the party reaches lvl 5 special qualities make their appearance (incorporeals, damage immunities and resistances, etc) and waving a pointy stick at them won't do. Enemies with special qualities become more frequent as levels go up and the fighter player accuses me of "targetting them." In addition, the adventure eventually grows into "ask the overdeity's oracle for the archfiend's true name, for which you have to go to a lost demiplane with access point in a sunken city under the sea," and all he can do is watch the spellcasters asking deities for the city's location, growing gills to swim, sky diving into elemental planes, and he can't contribute to any of that, so yeah, of course the player gets frustrated.
(well duh! It's a fucking ghost! Of course waving a pointy stick at it
won't work. Now stand aside and let the adults work.)
Games are not books, or anime. the Vanilla Action Hero is an archetype that shouldn't exist on any fantasy game, since all it does is mislead the players. If you play one in full awareness that you won't have any agency (or actually just play for the combat minigame), then more power to you, but if you like to have agency and making choices that matter, muggles are a trap option. Don't even bother.