Yes, gradual replacement is easier and more practical for large projects. However, typical C APIs and idioms are different from idiomatic Rust. Rust uses more type-system features, generics, iterators, RAII, and prefers tree-like data structures and far fewer pointers (no linked lists!).

C rewritten in Rust is still very C-like, and requires refactorings that you can't do until it's all Rust.

BTW: There are two GCC-based Rust implementations in the works, so compatibility with exotic platforms is going to be solved.