My issue is characters generally exists for main characters to surpass them, because the author spends more stock, the more time, the more effort and the more resources which time is also a resources in main character.
Swordsmen in One Piece are there for Zoro to surpass them either directly or indirectly. Black Blades exists so Zoro can finally learn about them and make one, just like Ryou exists for Zoro to learn and master it, like elemental swordsmanship like cutting fire exists for Zoro to learn and master it.
On top of it the concept of swords carrying will of swords master is very well established in manga and when Zoro acquired Enma, he in a way inherited Oden's will and as one of the main characters of the story he has to surpass the legacy of Oden, do what Kozuki Oden failed to do.
And one thing he certainly failed to do is as Hitetsu told us, turning Enma permanently black which Zoro would do, even if on the surface level Black Blade is nothing but a way to make your blade unbreakable, symbolically it is means of Oda showing us Zoro has graduated from being in Oden's league to a league beyond Oden regardless of specifics.
I know we have seen some crazy feats from Oden but in the history of One Piece what does Oden really represent? He does not represent peak of swordsmanship because the world did not recognize him as strongest swordsman in the world. He is not even regarded as the greatest samurai associated with Wano that is Ryuma. Oden's importance is linked strictly to this arc, he would not matter after Wano has ended, I don't think he would even be referenced.
So when Wano ends, it is essential that Zoro has graduated from what Oden represents so he starts working towards greater legacy and take next steps like surpassing Shanks then surpassing Mihawk then Ryuma then Roger or who ever else
@Celestial D. Dragon