You can absolutely use Win32 Forms and WPF (and probably Qt) with a game. They can be overlaid on top of a DirectX or OpenGL window (with a transparent background) -- I've done it before! I don't think it would have any of the downsides you mentioned, either, except that it wouldn't be GPU accelerated or actually rendered inside the graphics context, which is why nobody actually does this in practice.

But that ignores the dozens of GUI middleware specifically designed for games. A cursory Google search reveals that most of these are going to be retained mode. There's a reason for that. Projects like ImGUI appeal to mostly indie devs who don't have the time or resources to write their own GUI library or license some third-party middleware. And it's probably going to be just fine for their use case. But it's definitely not a perfect solution and we definitely shouldn't throw away decades worth of knowledge and experience like the article is implying.