I do not believe this. Most C code by far is not written in a way that makes it easy to formally guarantee memory safety, so the lowest effort path will necessarily involve refactoring it until it does.
And that means that rewriting it altogether into a new language has very few drawbacks, and some additional benefits.
Yes, you can rewrite everything in Elm, PureScript or even anything that is more similar to JS, but step by step conversion to TS enables a significant quality improvement for large codebases (I personally think it doesn't fare so well on small codebases, where there are some people who use it even for one-off scripts, but at least we can agree on large codebases I think)
I'm not sure that's what you meant, but interoperability between C and Rust does seem to be a selling point of Rust (like TS and JS), and allows C codebases to be oxidized gradually rather than rewritten from scratch.
By the way, I love rust, but I just don't understand the "rewrite all the things" hype. Being battle tested means a lot for low-level libs. You can't guarantee that you ported everything exactly, with bugs and all. Pragmatic solutions like "Checked C" prove a lot in that regard.
You actually can “just” turn C into Rust with something like c2rust. But you haven't gained any safety without refactoring.
Is that not also the case for Checked C?
No, because while some refactoring is necessary, it is virtually always local. E.g. if the sound checker can't prove that an access is within an arrays bounds, adding a dynamic test just before it (assuming you don't want to add annotations to help the checker) will make it provable.
Rewriting in a new language is certainly one way to reduce or eliminate memory errors, but if your goal is to reduce or eliminate memory errors in existing codebases, there are other ways that are cheaper, more proven, less risky and are applicable to more environments. These tools are already much more heavily used in safety- and security-critical codebases than any new language, so people are slowly getting good experience with them.