Unfortunately, this is where we run into an almost-surely irreconcilable conflict.
To you (and to me, and various others), there's nothing wrong with this. Someone can be "mundane" and still do things that are the stuff of myth and legend, because they're Just That Good. Such characters truly are still mundane. They've just refined mundane skill until it becomes something legendary, something that can spit in the eye of IRL physics.
To others--including several people in this thread--such an idea is not merely implausible, it is inherently self-contradictory. To be "mundane" IS to be absolutely, inherently incapable of feats of myth and legend. To be "mundane" is simply...to have really hard limits* and never, under any circumstances, any ability to exceed those limits. Exceeding those limits is what defines being supernatural, and exceeding them by simply accumulating enough mundane skill is a contradiction in terms, like saying one could assemble enough dryness to produce water or enough darkness to produce light.
As stated, I think this conflict is irreconcilable. Some folks just cannot, under any circumstances, accept "mundane skill that transcends the limits we normally ascribe to mundanity," even in a world where non-magical beings are doing things that aren't possible by Earth physics. In their eyes, there is a bright, hard, absolutely uncrossable line between "mundane" and "supernatural," and nothing, genuinely nothing whatsoever, can cross over that line without having an explicitly and inherently supernatural explanation for doing so.
Unfortunately, that group has almost always been both very vocal and very motivated, and often has had folks on their side actually doing the leadership of D&D design. As a result, their opinion usually wins out, and we're left with the unhealing wound of martial/caster disparity.
*Often, these limits are even harsher than the actual limits of IRL physics, as in, they're limits real live humans could break or have already broken. But that's a can of worms for another thread.
