> `final` doesn't buy you as much in Java as `const` does in C, or (the lack of) `mut` in Rust.

Quite true.

> `final` only helps you avoid dumb reassignment bugs that tests should probably catch anyway ...

There is another reason for declaring parameters `final` in Java. It is the only way[1] which they can be used in an anonymous class definition such as:

  return new SomeClass () {
    override int addOne () {
      return aNamedOuterMethodParam + 1;
    }
  }

1 - "The only way" as of Java 7. Newer versions may have removed this restriction, I haven't bothered to check.