The caller of the function knows nothing about what happens within the body of the function. (Is it just doing computation, or is it doing I/O?). The async keyword is how the author of the function makes it explicit that caller should choose when to await the result.
Isn't the alternative WCiYF is proposing to allow the caller to treat any function asynchronously, while having no way to discern whether doing so might be counterproductive?
Rust async functions are also different here. They don't run the code, they create a Future structure in it's initial state. Contrary to e.g. JavaScript, it doesn't start to run until you end up putting it on an executor. So the actual function call does something very different from what happens in other languages.
Isn't the alternative WCiYF is proposing to allow the caller to treat any function asynchronously, while having no way to discern whether doing so might be counterproductive?