Sure, you can't get completely perfect interop. However a LOT of real C++ codebases don't use or explicitly ban some of the trickiest features, so even partial support can go a long way. For instance, supporting the basic class/vtable layout C++ uses (or even just adopting the C++ ABI for Rust itself here) along with char* strings, would immediately allow you to call into a lot of real C++ APIs, albeit older ones.

Supporting C++ value types, even without templates, would in turn be enough to support quite a lot more. For instance Unreal Engine defines its own string types, as do Qt and so on.