They said they liked sum types, that doesn't automatically mean they want to learn Rust. I liked the STL containers when I read about them in the 1990s, but I don't like C++ and the existence of the containers did not change that.

Because I have a job and am not yet a dictator at my company for technical decisions.

Because Rust throws out the GC which means that you have to deal with complexities of lifetime management.

(I don't agree that sum types should be added to Go, as much as I dislike the language as it is)

As someone coming from C++ I don’t comprehend this complaint. How do you even do anything in a language without deterministic destructors? All my experiences with garbage collection have been painful. RAII is so much easier to use.

For resource management, Java/C# have try-with-resources/using statements. Python has `with`, golang has "defer".

GC addresses other issues like memory sharing, cyclic data structures, etc.