That's why you pair the immutable integer with a mutable slug. Nobody memorizes slugs so you don't have to worry about supporting the case of a user manually ty

That's why you pair the immutable integer with a mutable slug. Nobody memorizes slugs so you don't have to worry about supporting the case of a user manually typing in the URL from memory (which is about the only time when having a short integer in the URL would matter). With this scheme, you can change the slug at will, and in fact you can support loading the URL without having a slug at all, or with a made-up slug. This makes the URL resilient to minor transcription errors (e.g. accidentally deleting the last character of the URL, or screwing up when retyping it), and it also means anyone who's aware of this behavior can skip the slug entirely when retyping the URL.