That would have been a friendlier original design, I think it's past the point where they could implement this easily.
The Outfit system can be used to reskin any item as something else (turn that sword into a mace) but the Outfit system didn't always exist. And some people don't use the Outfits, or they use the Outfits but only apply styles to certain pieces because they're OK with the default item look.
Mechanically it would work yes. You could make the generic 1-h be a big foam bat for example.
But then what does that look like without the Outfit system? Now people who did it in the past have swords/daggers whatever but new players get the bat instead?
And what if they change directions and decide 1-h dagger and shield should play different from 1-h mace and shield? New players would have to farm again because their foam bat is none of the above.
I'm just saying yes if they made it like this originally (perhaps 1-h and shield was *always* a sword and shield) it would work, but it's complicated to try and make that change now.