This is from a much older part of the thread, but.... it doesn't look like a whole lot has changed, so....
Are they?
Can you build a Witcher in 5e's current class system. Eragon? The Knights Radiant (thematically, they're paladins, but ability-wise, almost all of them are Arcane Gishes)? Isaac and Hector from Castlevania (they cover their weapons in unholy fire that transforms the people they kill with those weapons into demons/undead monsters under their control)? Zuko from the last Airbender (a master of dual wielding swords that makes his attacks even better using Firebending)?
Can you truly build any of those without multiclassing so much that you no longer have a functional character?
I'm not saying that an "Arcane Warrior" class has to be able to make all of those different types of characters playable . . . but I am saying that there are character options in other fantasy settings that don't currently have a good way of translating them to D&D 5e.
Eh, I think the trouble with that is mainly that the trouble is theming and perceived power level.
The Witcher could easily be a Hunter or Monster Slayer Ranger re-themed to arcane instead of divine, a Valor Bard or Bladesinger Wizard re-themed to monster lore instead of Performance, or an Eldritch Knight basically as-is. Tell me that the
mechanics of the class actually fail here and not just the
theming of them. The Witcher is from a setting where there is
no divine magic. That doesn't mean the existing class mechanics don't represent what the character can do. It just means some of the theming is wrong. That's really just a refluff.
Additionally, Witchers are superheroic, all-around characters in the Witcher series precisely because they operate alone as solo protagonists. They don't translate well to D&D where you can't have a PC class do that. This has always been a narrative problem, because gishes are always experts at
everything. It's exactly why people roll their eyes at Drizzt or Artemis Entreri. They're faster, stronger, smarter, and better prepared than everyone else and that just doesn't work in a cooperative game where PCs need to share the spotlight and rely on each other for survival. That means it's easy to see why people don't like Eldritch Knight for a Witcher: because we all know it sucks and gets it's signature abilities
far too late to matter. It's a criticism of how poor EK is as a class because 1/3 casters are godawful gishes, 1/2 casters are only marginally better, and the full casters that still get extra attack have obvious things they gave up on. Unfortunately, when Superman and Martian Manhunter join the Justice League, they have to share the spotlight with Flash, Batman and Aquaman. (Which is precisely why people roll their eyes at Batman's power level, now.)
People don't just want to play a character that isn't just from an alien setting, they want to play characters from alien settings with
wholly incompatible magic systems. No, you probably can't make a elemental bender in D&D. Magic doesn't work that way in D&D. You probably can't be like an allomancer or a feruchemist from Mistborn, either. Brandon Sanderson has built a career on writing novels about wholly incompatible magic systems that all reside in the same universe. Magic is a lot different in Harry Potter, Dresden Files, Magic: The Gathering, Alex Verus, Seven Blades in Black, Lord of the Rings, Wheel of Time, The Blade Itself, etc. There are so many different ways magic can work. You're just not going to be able to put all of them into D&D. That's
not automatically a failing of D&D. D&D really can't be the everything game.
There are decisions that certainly do affect things that are worth exploring. The fact that concentration is crippling for melee spellcasters just fundamentally alters the viability of many different types of characters. Either way, it's not just about getting a new class. You'd need
a new system to make them really work. That's going to change the settings drastically.