The deepest inheritance chains I saw in practice were the OOP GUI programming toolkits, they'd go six-seven deep and invariable started needing to clone logic to avoid diamond issues (they were always single inheritance it seemed).

Honestly OOP did a lot better in those than the procedural toolkits they replaced in many ways. It was generally "better".

So if we're going to dump OOP for ??? I would say that a really amazing GUI toolkit that is pretty clearly better than the classical OOP inheritance model would really underscore the point. I can see a compositional interface GUI toolkit that is great.

I have since those days not done a native UI, so all of the modern ones, from KDE/Qt, GNOME, whatever the hell replaced MFC on windows, etc I am clueless about.

Plus all UIs are kind of dominated by HTML/CSS/javascript style of code, which is its own special evolutionary tree at this point.