General story telling. There’s no point him getting a new sword if it doesn’t eventually make him stronger.
But it’s also the case that Shusui was brilliant sword in it’s own right- we aren’t talking about Yubashiri here, we’re talking about one of the only two black blades we know about in existence, famed trasure of Wano, sword of Ryuma, equal in rank to Enma. It was a damned good sword.
Enma has forced Zoro to develop in a way that Shusui couldn’t have done, and we also know that one day, Zoro can make it even better- a supreme grade in it’s own right. But that day hasn’t came yet. Right now, Zoro is still showing difficulty in using it. He would not have that difficulty if he was using Shusui right now- but he also wouldn’t have the opportunity for growth.
Enma is, pardon the pun, a double edged sword. He will get stronger with it, but until it’s mastered, Zoro’s disadvantaged in a way that he wouldn’t be with Shusui.
Agreed, does not have to do with my question though.
We have no idea how the swords relate. Is Shusui that renowned because of its prowess or because Ryuuma is the one carried it? Not specified. It´s a great sword, but it mostly has value as Ryuuma´s inheritance.
Are black blades of same rank better or worse than their counterparts which are not black? No idea, iirc not specified either. Not every sword of same rank has the exact quality, and they each have their characteristics which can align with your strengths or be an hindrance.
I agree that Enma is dangerous to use, but here is the deal. It can bring out a lot of AP, much more than you had before, even without mastering it completely. The mastering simply entails that you can control it fully, and decide how much and when you use the strength you want to use in that exact moment, basically a more advanced problem than Zoro already had with Kitetsu.
Having said that, nothing implies that the potential AP Zoro can bring out of Enma even without fully mastering it, is below that of Shusui, only thing that is implied is that with Shusui he was in complete control.
And that is indicated to me in Enma´s introduction, Zoro aimed at cutting the tree, somethign he would have done so with the other swords, but Enma forced out much more AP and cut the hill, only it was detrimental to Zoro and his Haki.