Since when has static typing eliminated backward-compatibility problems in interfaces? Not all visible runtime behavior can be encoded in the type system, unless you're using a proof assistant or formal-verification system.
Even formal verification misses visible behaviors, the line is just a little farther down. Correct runtime visible behavior can also include side effects like the amount of time taken (e.g. constant time cryptography), or even the amount of heat generated [0]. You're not going to encode everything you could possibly want in a type system without modeling the entire universe, so draw an opinionated line somewhere reasonable and be happy with it.
