I'd like to point out that the only operation we can do on the first
argument of MkFoo is to show to it. This is all we can ever do:
we have no idea of its type but we know we can show it and get a
String.

That's not all you can do: you can also pass the first argument of MkFoo to a function that expects a Show a => a argument, like the function 'print'. Can you do that with just a String (that represents show x for some x)?