Yes but my point is that if you allow an instance of a specific class to either have or not have a specific method/behavior at runtime, then you lose type safet

Yes but my point is that if you allow an instance of a specific class to either have or not have a specific method/behavior at runtime, then you lose type safety which is the whole point of a dynamically typed language. The point that I'm trying to make is that true compile-time type safety is not possible in my example without some kind of rigid complex inheritance structure that is predefined at compile time.