Most of them. Very few frameworks are actually written primarily in Swift.
And if so, what benefit would it provide?
But pieces absolutely could be. The benefit would be compile-time checking for memory safety issues (reducing crashes) and language-level concurrency (fewer race conditions and a much easier path to parallelize single-threaded code for performance).
It sounds to me like Apple would like Swift, perhaps with some extra tooling, to be able to handle the Mach kernel. I don’t know if they’ll go that far though.