Even if every project can only be 90% done, that’s a huge win. Best would be if it could just wrap the C equivalent code into an unsafe block which would be automatically triaged for human review.
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.
c2rust exists today, and pretty much satisfies this. I've used it to convert a few legacy math libraries to unsafe rust, and then been able to do the unsafe->safe refactor in the relative comfort of the full rust toolset (analyser + IDE + tests)
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
"real" (large) C/C++ programs get much of their complexity from the fact that it's hundred of "sources" (both compiled and libraries) that sometimes, or even often, share global state and at best use a form of "opportunistic sharing". Global variables are (deliberately, and justifiedly-so) hard in rust, but (too) trivial in C/C++, cross-references / pointer chains / multi-references likewise. And once you enter threading, it becomes even harder to output "good" rust code - you'd have to prove func() is called from threaded code and should in rust best take Arc<> or some such instead of a pointer.
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".
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.