By removing memory safety bugs, it allows reviewers to focus on the more important sources of vulnerabilities - the things that the compiler can't check. On that front, it is also worth noting that Rust has a much richer type system than C or C++, and can be leveraged to craft domain models that catch many more bugs at compile time.

With all of that said however, I agree with you that it would be foolish to think that a rewrite would not be open to lots of potential vulnerabilities. At the moment might be prudent to continue to use and support the current libraries, but to also support efforts like rust-crypto in order to prepare for the future.