Balance. Why do we need it?

[Knaight]

When the Balrog came, it wasn't some Human Fighter who prevented its passage, nor who smote its ruin upon the mount.

Meanwhile Feanor cut down somewhere in the vicinity of a dozen Balrogs with a sword.

Both of these have at least some divinity in their make-up though, so they're not exactly representative. There is no "plausibility gap" when a half deity is easily beating up a spellcaster. In D&D terms, these guys would almost certainly have a template of some kind, if not being a special race entirely with a boatload of LA.

So did Gandalf, in the other direction. But fine, we can lower the bar from whomping on them effortlessly for days to just winning, which opens the field to a lot of characters. Hurin & Turin from The Silmarillion; Lancelot, Gawain, Owain, Percival, Bors, and Yvain (who to be fair is basically another iteration of Owain) from Arthurian myth; basically the entire main cast of Water Margin (although the mages in that are a cut above); basically the entire main cast from Romance of the Three Kingdoms; the list goes on.

The depiction of wizards as people who are specifically better in a fight than actual warriors isn't anywhere near universal in fantasy, particularly if they get ambushed.

Even limiting myself to your arbitrary "pre-D&D" stipulation there are indeed plenty of powerful magicians lacking the kind of divine origin that Hercules and Cu Chulainn etc have like Prospero, Abeno Seimei, Nicolas Flamel, the Witches of Oz etc. We also have D&D's contemporaries (i.e. mages that were unlikely to have taken direct inspiration from it) like Feist's Pug and Dahl's Matilda. Then of course we have D&D's own influences like Jack Vance.

There's also plenty of fictional warriors who have no divine origin but are still utterly terrifying, starting with most of the later additions to the Arthurian myths.

As for Jack Vance, a wizard in Dying Earth is exceptional if they can hold a whole three spells at once, and legendary if they can manage six. There's also a serious case to be made that in terms of getting things done the most effective character is Cugel, who is a trickster first and foremost.