Not OP, but several domains have large non-trivial C++ codebases. For example, numerics, computational geometry and computer graphics have large mature librarie

Not OP, but several domains have large non-trivial C++ codebases. For example, numerics, computational geometry and computer graphics have large mature libraries that would need to be rewritten by subject experts. The number of subject experts in these fields is not that large to start with, and the algorithms are complex and non-trivial, so "exothermic" explosion to new languages like Rust is unlikely to happen very fast.