Zoro has to trade in his strongest sword, Shusui, for Enma. They're also both the same grade while Shusui is also a black blade. You'd have more of an argument if he traded in Sandai Kitetsu, and instead currently had Wado, Shusui and Enma.
Zoro's arm is drained when first using the sword. Then he tames it and has to train with it in order to get stronger. I like this comparison others have used so I'll mention it too. If you went to the gym, and built up muscle, would you credit the results to the weights or your own hard work?
Enma is simply a tool that serves as a means for Zoro to power up through hard work and training. It's not a free power up and any increase in power Zoro gets would be a result of his actual training with the sword, not the sword itself.
The only other argument I've seen is that Enma is a Great Grade blade in base while Shusui is a Great Grade blade after being blackened. Yes, Enma as a sword holds more potential than Shusui, but even Hitetsu says this is dependent on Zoro
So when it happens, he will have earned it through his own merit.