Assuming there's a good reason to do this (there isn't) and assuming that all the code can actually be replicated in Swift (it can't), it would literally take decades to sift through all the code in OS X, rewrite it in Swift, test it, debug it, test it again, debug it again, then test it with every in-house and 3rd party application, combinations of applications, debug that, test that again, etc. There wouldn't be any benefit either, because it works just fine as it is.
Quote from Apple:
"Designed for Safety
Swift eliminates entire classes of unsafe code. Variables are always initialized before use, arrays and integers are checked for overflow, and memory is managed automatically. Syntax is tuned to make it easy to define your intent for example, simple three-character keywords define a variable (var) or constant (let).
The safe patterns in Swift are tuned for the powerful Cocoa and Cocoa Touch APIs. Understanding and properly handling cases where objects are nil is fundamental to the frameworks, and Swift code makes this extremely easy. Adding a single character can replace what used to be an entire line of code in Objective-C. This all works together to make building iOS and Mac apps easier and safer than ever before.
Fast and Powerful
From its earliest conception, Swift was built to be fast. Using the incredibly high-performance LLVM compiler, Swift code is transformed into optimized native code, tuned to get the most out of modern Mac, iPhone, and iPad hardware. The syntax and standard library have also been tuned to make the most obvious way to write your code also perform the best.
Swift is a successor to the C and Objective-C languages. It includes low-level primitives such as types, flow control, and operators. It also provides object-oriented features such as classes, protocols, and generics, giving Cocoa and Cocoa Touch developers the performance and power they demand."
Specifically this:
Swift is a successor to the C and Objective-C languages. It includes low-level primitives such as types, flow control, and operators.
Swift is a successor to Obj-C and seems to have many benefits. A company such Apple shouldn't have any issues rewriting there OS, and testing won't take decades. A bit of an exaggeration....