> When we say "A extends B" we mean a) an instance of A composes in an instance of B b) A implements the interface that B implicitly expresses c) this implementation, by default, delegates to the instance of A from a).

This is probably the best summary of why traditional OOP is often suboptimal I've read. Clearly OOP solves (or solved) a problem that people were having, it's just that it tries to solve too many problems with the same system.

It's also worth noting that fans of "composition over inheritance" usually don't notice or mention that if you go with pure composition, you lose out on point (c) in most programming languages. Very few languages have a built-in mechanism by which a set of operations on type A can be automatically delegated to one of its elements, without at a minimum, listing all the operations to be delegated. So at least at present, traditional inheritance still provides an operation which is not available with composition.