Depends on which version of Merlin you're reading.
I'm thinking classic version. Like most wizards or the like in legend, Merlin was a seer. In some versions he engaged in creating illusions or shape-changing, powers commonly attributed to Celtic Druids, from which he was likely derived.
He didn't throw balls of fire or bolts of lighting or turn into an Anis hag and rip knights to pieces or anything remotely along the lines of what D&D wizards do on a daily basis.
Knights did not try to fight Merlin. Merlin seated a king on the throne and managed him. Merlin considered himself a manipulator of man's affairs. He knew men would follow someone like Arhtur they understood rather than someone like Merlin that they didn't. Everyone feared Merlin.
Of course, consider that none of that required magical powers. Knights didn't challenge old men to duels. Cunning manipulators have put men on thrones, been the power behind them, and engendered fear in the community without having any magical powers whatsoever.
It is that relative fear that I believe D&D tries to foster through its system.
That's where it tries to go with monsters and other antagonists, sure. PC wizards & clerics probably shouldn't be inspiring that sort of reaction from their allies.
I admit 3E took the caster-martial disparity way too far in the caster direction. 5E is a much better wheelhouse for the disparity.
But, it's still a disparity. 5e aimed to be lack the classic game, the classic game was imbalanced, but could be slapped into shape by the DM. So, 5e doesn't bother with balance, but does push DM empowerment.
If either one obviates the other, then you lose something in the genre. What would Merlin be without Arthur? Or Gandalf without Aragorn or the Hobbits? Gotta have that dependency to drive the story meaning everyone brings something to the table that is needed.
Nod. Those are the 'iconic roles' that D&D has always had. Fighter, Cleric, Magic-user, Thief. In 3e, Tier 1 classes did obviate the others. In 4e, they were neatly balanced, no class obviated another, and party interdependence and synergy were well supported. In 5e, the iconic 'big 4' classes still have their traditional roles, but some of the other classes are more questionable - and the emphasis on casting is overwhelming. Every class uses magic. Even the fighters cast spells...
It's interesting. Back in the 3.5 era, on the Gleemax, I think the Wizard's form was, at the time, there were a lot of "Fighter SUX" threads. One of the things it would come down to, from my PoV, was that the nature of magic vs martial in traditional D&D - the former being limited-use and powerful, the latter unlimited and underpowered - made balancing the classes impossible. Doing so would require either re-designing classes to all be like the fighter (the 3e fighter was a neat, customizeable design) or all be like Vancian casters. Both seemed unthinkable.
5e has very nearly taken me up on that last alternative: Every class has at least one sub-class that uses magic, and virtually all of those, including a fighter and thief sub-class, do so by casting spells.
The fighter brings very reliable, consistent fighting prowess with DPR and durability.
Yeah, prettymuch back to the 2e model. Except for the EK alluded to, above, of course.