I'm A Banana
Potassium-Rich
How's that? There's nothing inherent in "Fighter" or "Wizard" that is unique to it that is essential to party survivability. And various gish-type characters that blend elements of both are quite common (especially in 5e!).But if you try to play either as the other you're gonna die.
Fighter used to be pretty coherent. And Fighter used to be Fighting Man.
The OG three-class structure was intentionally rather generic. Fighting Man, Magic User, Cleric; Martial, Magical, and a Gish. The first two especially were very much "all knights, mercenaries, soldiers, warriors, archers, pikemen, barbarians, etc. are Fighting Men; all sorcerers, enchanters, illusionists, necromancers, fortune-tellers, sages, wise men, witches, etc. are Magic Users"
But over time, "Magic User" became "Wizard" and became a specific type of bookish, intellectual spellcaster, making room for all sorts of other magical people.
"Fighting Man" got broken up into paladin or ranger or barbarian or even rogue...
And we don't have a generic "Magic User" class anymore - we have a bunch of specific spellcasters.
But we still have a "Fighter." They trundle on as a "generic" class trying to compete in a crowded field of characters that are a lot like a Fighter, but with more specific interesting detail.
The Fighter's lack of specific narrative traits is a significant part of this martial/magical problem. Which is where more specific character archetypes help. Dragon Slayer or Aberration Hunter trumps "Fighter" in terms of the neat stuff we'd imagine them doing.
Party comp building is very 4e!One thing I've borrowed from WFRP 3e and Blades in the Dark for my 4e retroclone is the idea of a "Party Sheet" where the party agrees what they are about and get some bonuses for it. So a pirate crew is using a different sheet from a band on the run which is again different to a group trying to secure their home village and possibly become the local lords.
But, like, if I'm a tanky fighter build or a heavy-armor cleric build and I'm part of a pirate crew, we've got a bit of an issue what with heavy, rusty metal armor in the fiction of this salt-and-sea kind of narrative. Not really the same story.
Or, if I'm in a band on the run and I choose a character who is into crafting and needs things like a laboratory or library or other craft station.
Or, if I'm in a group trying to defend my home village and I choose an interplanar wanderer or a character of the wilderness like a druid or ranger.
Similarly, if I'm Grim Darkblade, a gritty warrior scarred by the horrors of war and I'm in the game where we go to the Feywild and talk with bunny rabbit people, I'm gonna have a bad time.
So, yes, party comp. But maybe let's only include options in the pool of selectable characters that are relevant for the adventure. Which may mean that a purely martial class just doesn't vibe, in the same way that a class with heavy armor or a crafting need or who can survive in the wilderness may just not fit. If you're telling a story about heroes in a world of magic, maybe "Doesn't Use Magic At All" isn't really a valid option most of the time.