Just getting something vaguely Rust shaped which can compile is the first step in overcoming the inertia to leave the program in its current language.
There is real utility in slowly fleshing out the number of transforms in a tool like c2rust that can recognise high-level constructs in C code and produce idiomatic safe equivalents in rust
It'll be great for "pure" functions. For the grimey parts of the world, funcs taking pointer args and returning pointers, for things that access and modify global data without locks, for threaded code with implicit (and undocumented) locking, the tool would add most value. If it can. Even only by saying "this code looks grimey. here's why. A bit of FFI will also be thrown in because it links against 100 libraries. I suggest changes along those lines ... use one of the 2000000 hint flags to pick-your-evil".