Memory safety is just one aspect where the compiler can help making sure a program is correct. The more the compiler helps with static analysis, the less we need to rely on creating tests for edge cases.
I feel as though not enough attention is given to how std is designed. For example: [u8], str, Path, and OsStr may be confusing at first, but when you understand why they are there any other approach feels icky. std guides you down a path of caring about things that really should matter (at least if you're only unwrapping provably safe values).
Have you considered what happens if not-utf8 data winds up in an environment variable that you are writing to stdout? What if it contains malicious VT commands?
Unless you're talking about terminal bugs in parsing invalid UTF-8 - and parsing invalid UTF-8 is easier than rendering valid UTF-8 - VT commands are UTF-8 compatible. You just need to embed an ASCII escape character.
If an algorithm works for invalid Unicode, it should probably be an algorithm on bytes, not strings.