Not sure what you are getting at. If you add something to the interface of Awesome, then any call site which passes a thing not having a PerformAwesomeness() method will fail to compile.

Which means that every time you change an interface you have to compile everything (given that you don't explicitly call out the Awesome interface anywhere, so you don't know whether any particular source file will require the Awesome interface without compiling it).

Unless you rely on introspection, which happens a lot when your language does not support generics.

No, it really doesn't happen a lot. If it does, you're doing it wrong.

If you don't have parametric polymorphism, you only have two options to implement new containers: a new implementation for each type or using interface{}. The first leads to a lot of duplicate code, the latter requires you to use introspection to use safely.

So, it does happen a lot, or you are making duplicate code.

Or you just don't need polymorphic containers that often (other than the ones that go already gives you).