I voted no and I have both a story and gameplay reason for it.
Story wise, so far it seems like losing his arm is kind of gonna be the crux of his potential subplot with Vergil. I think that event would lose its impact if he gets the Devil Bringer back, and would feel pretty cheap imo.
Gameplay wise, hey, the Devil Bringer was definitely one of the most fun things to play with in DMC4, especially against bosses but let's be honest, these Devil Breakers look a lot more fun and vary than his old arm ever was. And I dare say ever could be too, as I don't see Nero being able to surf on his Devil Bringer, or shoot a barrage of lasers with it, or turn it into a freaking railgun, etc etc. The very recent news that we'll be able to freely switch between these Breakers in the full game is all I needed to say that they are what Nero needed to make his gameplay more vary and his moveset more versatile.
So to answer OP's question, yeah, I think giving Nero his Bringer back would indeed be a downgrade.
That said though, I wouldn't mind it if they gave him a separate special costume where you get to play with his old DB, with a dedicated moveset. They did it once in DMC3 with Vergil's corrupted costume in Nelo Angelo mode, they could do it again. His moveset was also more limited and different than regular Vergil's but it was a cool feature and the game was still playable with it.