The obvious question is, how many of those were the sort that writing in a memory-safe language would make impossible?
They should prompt one of the more adventurous LLMs to find security bugs and with some luck it will deviate from the prompt and rewrite ffmpeg in Rust.
Rewriting ffmpeg in Rust will not solve it. The parts that are memory unsafe in ffmpeg, and similar projects, are not unsafe because C or C++ is inherently unsafe. Instead, it's the CODE that is unsafe. Translating the code (data structures, logic, etc) to Rust does not fix bugs in the code. That code will be littered with "unsafe" code, and of course, it will no longer be maintainable.