> Can all the maintainers of the c codebase move over to maintaining (forward) the Rust codebase. Surely there'll be some friction, and losses to friction.

there are always losses. Anyone who can maintain a non-trival C code base can maintain Rust. It will take them some time to learn, but learning a new programming language is not hard for someone who wants to. In a couple years they will be just as productive.

The question is do they want to? I do not have much hope the existing maintainers will move. Some will, but I expect the majority will not.