A more interesting martial class, with more and more unique options would be a great addition to the game, opening up modes of play that aren't currently available, including some that were available in 3.5 and 4e, which is in keeping with 5e's goal of being for fans of all prior editions.
If the argument had been made instead that spellcasters have even more options than fighters do, I wouldn't try to debunk that argument, I'd agree with it! It's why Eldritch Knight is clearly the best kind of fighter IMO, and why Paladin/Sorcerers are superior to Paladins. In my subjective opinion.
But that isn't the argument I was responding to.
There is a current running through this behind the scenes, which has been with us for as long as D&D has existed. The magic as technology debate.
Now magic, in fantasy literature, was originally the province of bad guys, or at least dubious ones. In pulp fiction the heroes were bronzed, strapping lads, with lightning reflexes and iron wills who could barely tolerate the existence of thin, pasty, "lab rat" sorcerers with their yellowed eyes and fuming potions. So much so that Micheal Moorcock deliberately inverted the cliche to give us Elric of Melniboné as a hero who was a skinny, albino, sorcerer who needed drugs to be strong. And early D&D was strongly informed by these pulp tropes, so much so that the original alignment system was Law vs Chaos (with Neutral in the middle), the same axis of conflict that drove Moorcocks Eternal Champion books.
OTOH in a lot of other fantasy genres, and increasingly so in modern fantasy, magic loses it's lustre of evil. It may be commonplace like in the Lord of the Rings (where low level magic was everywhere but combat magic was exceedingly rare), or vanishingly rare like in the Belgariad where only a handful of people ever had access to it, but it does not taint or demean the possessor of it (with rare and specific exceptions, like the One Ring, which was created for that purpose.) A magician in these worlds does not pay for his powers with his soul, or lose his mind to the slow corrosion of madness. He simply has more and better options, or power than a mundane man, just as a man with a sword has more options in a fight than a man without. Magic is technology.
And D&D is also informed by these stories. And it has leaned more and more strongly in that direction over the years. It is a fantasy game after all, and who hasn't fantasized of magical powers? Of flight, and invisibility, shapeshifting and reading minds.
So there has been tension between the use of magic as a moral device where magic is the quick, easy and treacherous path to power ala the Dark side, and magic as wish fullfilment or alternate physics.
And there has also been the question of the role of the rules in a roleplaying game. Are they there to describe the world, and allow simulation of the conflicts within it? Provide narrative elements for storytelling withing the fiction of the game? Provide balanced and interesting puzzles for players to solve? The different editions of the game have given very different priorities to these different aspects of playing, just have different players have distinct preferences.
Now, in the current edition of D&D there in nothing inherently sinister about magic (with the probable exception of Warlocks), so it falls into the Magic as Technology camp of world building. The trouble is you now have conflict between the different goals of rule design, and the differing desires of the players. For some people, informed by those early tropes or just a desire to play the under dog, will always want to play the hero who succeeds without magic. However if the rules are set up to make the "rules as physics" camp happy then a mundane character is at multiple disadvantages to the magic user. Because magic is a tool, and tools both provide more options, and multiply power to allow more to be accomplished with less effort. So really, a spellless fighter makes as much sense as someone who wants to serve in the modern military as a soldier without using technology. A man with a gun is more dangerous than a man without one. Tools are options, and the magicless man has elected to leave his toolbelt at home.
The other camps of rule design don't care. They want the tools to tell the story they want to tell, or they want all characters to have equivalent tools to use to solve the (tactical) puzzles of the game without being hindered by the "fluff".
The rules as physics aspect of game design was the one that 4e left out in the rain, and so 5e had to bring it back into the mix, which it has, as a consequence of which the mundane does have fewer options than a magic wielder. I think 5e strikes a pretty good balance between the different goals of design and play, but then I'm a rules as physics guy so I'm biased.