Idk, you should ask the maintainers these questions, or the Ubuntu maintainers. I'm not particularly arguing in favour of this rewrite, but the title and contents of the post are talking about Rust in general and the type of bugs it can/can't prevent.

Perhaps one good reason is that once the initial bugs are fixed, over time the number of security issues will be lower than the original? If it could reach the same level of stability and robustness in months or a small number of years, the downsides aren't totally obvious. We will have to wait to judge I suppose. Maybe it's not worth it and that's fine, but it doesn't speak to Rust as a language.