> Go should really have an analogue to Rust's `?`, but you can't really do that in a sane way without sum types

It could be just some simple (hey, that's what go wants to be, right?) macro thing, that just does the everyday `if err!=nil{return ..., err}` for you without having to juggle (and think about) vars.

    b := g(f()?)?
    // var b B
    // {
    //     var a A
    //     var err error
    //     a, err = f()
    //     if err != nil {
    //         return *new(A), *new(B), err
    //     }
    //     var b B
    //     b, err = g(a)
    //     if err != nil {
    //         return *new(A), *new(B), err
    //     }
    //     // no accidental reuse of err
    // }

For a more corporate driven and controlled language, once it goes down a certain direction, it appears really hard for it to stop the train or get enough momentum to change directions. Easy examples are generics, enums, sum types, etc...

Therefore other languages (influenced by Golang), like V (vlang.io), Odin (odin-lang.org), Jai, C3... made such improvements. One could argue that it's more a matter of if people are going to be brave or open-minded enough to try something different or go in a different direction.