You assume that the only way a language can help you write safe code is by making sound guarantees; this is wrong. A language can also help you write safe code

You assume that the only way a language can help you write safe code is by making sound guarantees; this is wrong. A language can also help you write safe code by making it easy to selectively add runtime checks (that can then be selectively turned off in production), by making code-build-test cycles faster, and by making it easy to inspect what the code does. This approach has tradeoffs: it is unsound so it will let some undefined behavior bugs through; on the other hand it might do more to reduce bugs that are unrelated to undefined behavior.