No new code goes in that violates the rule, and ideally no code at all goes in that is both unsafe and parses untrusted data (regardless of sandboxing) and old code doing both gets replaced.
A giant pile of C++ can be used for rendering, not parsing untrusted data. A giant pile of C++ can sit behind a validator: a memory-safe JSON validator can vet a stream, before an C++ library deserializes it. Etc.
