Consider a bit of a different case. I run a service that exposes an API, and some fields in some response bodies are enums. I've published a Rust client for the API for my customers to do, and (among other things) it has something like this:

    #[derive(serde::Serialize, serde::Deserialize)]
    pub struct SomeEnum {
        AValue,
        BValue,
    }

My customers use that and all is well. But I want to add a new enum value, CValue. I can't require that all my customers update their version of my Rust client before I add it; that would be unreasonable.

So I add it, and what happens? Well, now whenever my customers make that API call, instead of getting some API object back, they get a deserialization error, because that enum's Deserialize impl doesn't know how to handle "CValue". Maybe some customer wasn't even using that field in the returned API object, but now I've broken their code.

Adding #[non_exhaustive] means I at least won't break my customers' code when I add a new enum value.