Whether that's true or false depends on what your hypothetical developer wants to do in the future. When an all-new driver is written in Rust, then working on that driver will require knowing Rust. But nobody's talking about trying to re-write all the existing drivers in Rust, so there will still be plenty of C code to work on even in the DRM subsystem.

I think this is true. But that'd be nonetheless up to the subsystem maintainers.