Is inheritance in Java overrated ? Needed?

The real answer is no. Inheritance is absolutely unnecessary for API for GUIs. Absolutely unnecessary. If we are talking about GUI callbacks, what you want is a type systems that's statically-typed, with functions as first-class citizens (Hindley-Milner type system preferred) and with records (which are syntactic sugar anyway, they are just glorified tuples). Put a bunch of functions into a record and there's your GUI callback. This is a much cleaner and simpler way of doing it.

Inheritance is a fundamentally flawed concept in type systems. It suffers from the fragile base class problem. Josh Bloch, writer of Effective Java, recommends using composition over inheritance. If you want inheritance so that you can selectively override event handlers, and fall back on default implementations... you want something that's broken, because that suffers from the fragile base class problem. You need a guarantee that the superclass you are overriding won't call your subclass methods in an unexpected way... in other words, you need implementation details. A better way is to explicitly call default handlers on the superclass.