I buy the argument that the Finder was written in Carbon in order to eat the Carbon dog food and to shake out the OS X implementation of Carbon. However, I still stand behind my statement that they wouldn't have engineered a critical piece of OS X in Carbon if it were an interim API and doomed to go away. We are probably going to be using this Finder for a while.
I'm actually a bit bummed by the Finder being in Carbon and probably remaining that way for a while. It would make a lot of sense for Cocoa services to be exposed to items in the Finder. Quick encryption and signing of files comes to mind. Of course maybe Apple will provide a way for that to happen.
As to Apple jettisoning one or the other: I don't see it happening. If anything, I think that the reuse under the covers of Carbon and Cocoa will only continue to improve.