[throw10920]

This is fallacious. You could use the same logic to argue that we should encode the type of every argument and return value of a function into the function signature, and have to explicitly write it out by hand at every call site, for the same reason:

x = number:foo(number:x, string:y)

It's absurd. The type system should responsible for keeping track of the async status of the function, and you should get that when hovering over the function in your IDE. It does not belong in the syntax any more than the above does and it's an absolutely terrible reason to duplicate all of your functions and introduce these huge headaches.