I think there is significant merit to rewriting a legacy C++ (or C) codebase in very modern C++. I've done it before and it not only greatly reduced the total amount of code but also substantially improved the general safety. Faster code and higher quality. Because both implementations are "C++", there is a much more incremental path and the existing testing more or less just works.

By contrast, my experience with C++ to Rust rewrites is that the inability of Rust to express some useful and common C++ constructs causes the software architecture to diverge to the point where you might as well just be rewriting it from scratch because it is too difficult to track the C++ code.