Zoro doesn't need a sword that can absorb his opponent's haki. Swordsmanship is already one of the strongest fighting styles in the manga and has the potential to be the strongest.
Giving a sword that power is turning a sword into a DF. We already know you can add DF powers to a sword so that is not necessary.
What's a stronger defense than a sword that can't be broken? A swordsman with a black blade has the potential to have an unbreakable defense.
I've talked about the potential a swordsman has from creating and possessing a black blade in another post.
Here it is:
Combine with the Pinnacle of swordsmanship, the ability to cut anything, and they are a walking irresistible force paradox. A sword that can cut anything vs a shield that can't be broken. With that power who can stop them.
I still don't think people truly understand the potential swordsman have One Piece.
Zoro already has a sword that absorbs haki. If you don't want at least one of Zoro's swords to behave as though it's alive that ship's sailed I'm afraid. There's an opportunity for real depth and lore to be added to one of the most popular fighting styles in the story and you'd rather black blades to just be a stronger material? There's no way this unique disposition in blades stuff is ending here, and if that means a few characters in the entire story get further abilities is that such a bad thing?
That being said you are right, swordsmen have the privilege that few other types of fighters have as they can typically one-shot their opponent. But that doesn't change the fact that Zoro isn't at all versatile outside of that, black blades being unbreakable isn't helpful as armament at Zoro's level achieves that already.
For what you say about Zoro/swordsmen to be true their opponent has to have only defence, he could barely handle fighting onimaru and killer at once so what good is the ability to cut anything. You say a black blade is the best defence, yet Zoro isn't able to block every attack with his swords so who cares, is my line of thinking.
What powerups do you see Zoro having in his future, aside from the obvious haki improvements. How do you imagine his and Mihawk's last fight looking like? Oda rarely has Zoro's matchup end up as just a slashing contest, all of his big fights have another dimension to them.