Again these discussions are backward.
If the fighter is supposed to fight, lok at who he's fighting.
A CR 11 Horned devil has
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That would be 4 Level 11 fighters, right? A single CR 11 monster should be an 'average' fight for a party of 4, correct?
I'd give that fight to the 4 level 11 fighters, just using mundane equipment.
A lot of this debate depends heavily on how the game is run, the encounter distances, what other environmental factors are at play, etc.
I just finished up a campaign where all the characters were spell casters of some sort or another, or MC'd into casters. It was a total disaster. Every encounter, unless I specifically tailored it to counter their magic and other sundry abilities, was a cake walk. The pillars (explore, social)? Use a spell (detect magic, charm person, detect thoughts, etc.). Wear 'em down? "Don't waste spell slots on these, just use cantrips!" was the refrain.
I had one straight fighter who started in the campaign, but dropped out due to IRL stuff. He really enjoyed playing a straight fighter using the 5e playtest rules with superiority dice, and the choices that class/build option offered. So I let him try it out.
He also did a bang up job getting in the front of stuff, but was also the only one who bothered to approach social situations, engage with NPC's, and subsequently earn Inspiration for various things.
So "mundane" fighters can work as long as the campaign supports them, they have opportunity to do stuff outside of just swing swords, etc. At least in my experience. I also wasn't stingy with magic items/swords, but he kept giving them away, or burying them with important NPCs or nobles whose tombs the party ransacked. It was pretty hilarious.