Thank you for such elegant and laconic explanation, sir 
I have one more question: is it worth to create any mechanism to describe that entity can't conform some protocol or be derived from class?
If such thing existed, it would be easier to add constraint to keep hierarchy more safe, because now it's very easy to shoot yourself in the foot with it and Xcode wouldn't warn you about it.
What do you think?
@anandabits your opinion also is very interesting for me