Well, he can focus all his training efforts into that arm. XD
Seriously though, I'm not sure if he ever will have gotten the prosthetic arm. He's very proud of his lineage and the thought of having more Senju cells might not be for him.
They had to nerf him for the future. Sasuke is the only one with the potential to surpass the sage of six path. XD
Real Talk...
He left the village in manga 699.. the movie happen 2 years later. Meaning he was gone for 2 years. So he probably will get his hand at end of the movie.
I'm sure he'll get his arm at the end of the movie. The thing is, he'll have trouble with fighting and won't be able to use many jutsu. Obviously it's a pretty big nerf, makes it even more interesting.