> In other words, shouldn't the "rewrite it in Rust" folks have to prove that the cost of their proposed rewrite will be justified?

Agreed, they do. But that it hasn't been proven to make things better doesn't mean it will make things worse. It just means that we don't know enough to say. The right way to answer the "would this software have fewer bugs if it were rewritten in Rust?" question requires a detailed look at what bugs the software has empirically encountered.