There is no language feature for a tree map. Many problems require a tree map. Either inclusion of generics was a good thing, or it is worth sacrificing static typing or requiring codegen for these use cases. Repeat for numerous collections and APIs.

Disallowing someone from using easy statically typed tree maps is not accomplishing any of the simplicity virtues people trumpet Go for having. While the much-warned-of castles of inappropriately applied generics have yet to be found in any codebase I've worked with in any language, including Rust.