Yeah, projects also exist in the real world and practical considerations matter.

The legacy C/C++ code base might slowly and strategically have components refactored into rust, or you might leave it.

The C/C++ team might be interested in trying Rust, but have to code urgent projects in C/C++.

In the same way that if you have a perfectly good felling axe and someone just invented the chain saw, you're better off felling that tree with your axe than going into town, buying a chainsaw and figuring out how to use it. The axe isn't really the right tool for the job anymore, but it still works.