Why would you want to cast mixer to something else? What real-life task does it solve?
> If we assume we have both a hardware mixer and a soft mixer which both have the set volume method, we wan't polymorphism so we can treat them equally.
Polymorphism isn't OOP feature. You can have overloaded `mixer_set(struct, int)` without any OOP wizardry involved.
Did you ever heard about interfaces? You can have a IMixer interface, a Mixer baseclass and some concrete mixer classes depending on your task. Like null-mixer for mocking, software mixer for legacy computers, hardware mixer with GPU support, etc. Use your imagination, Luke!
Now you can do this without OOP or making a huge chunk of form code, but OOP is a natural fit. Especially if we may want to add pictures or video in the future.
That would work without OOP. Make a FormRender component that manages that logic (being rendered in a form) and other components, one that manages a keyboard, other that can be clicked and handle hover, touch states, etc.
IMO, the problem is it works really well in so many contexts that people end up trying to use it everywhere.
As to messaging, consider what happens when you add JavaScript that can change the DOM. Now, you need to pass messages in both directions not just What is your Height- Width, but also my Height-Width has changed. Animated Gifs or Videos now want to update things in real time, and other windows can cover parts of your view port.
And let's not forget about transparency.
PS: ECS can still work, but it starts getting really complex.