Yes? Why not?

What's the objective benefit of rewriting something already working into a different language?

You could or not, it's a tradeoff. Not sure what your criteria for "objective" is. In iOS-land you gotta keep up with the changes every year or your app will show its age. You could continue to do this in ObjC year after year but it's increasingly tough to hire ObjC devs, and Swift offers the possibility of a gradual transition rather than a full on rewrite. Not to mention the quality of life improvements.

The ability to hire developers to work on your product?