Seems to me like immediate mode UIs were dismissed rather too quickly. Why not update a DAW UI like a video game? It has similar realtime requirements.
The fact that he's designed a retained mode library with an immediate mode API doesn't mean that it doesn't have literally the same limitations, because he doesn't expose the abstractions to you to actually do meaningful retained work.
I don't care what made up paradigm someone else has invented.
Anyone who was doing immediate mode graphics programming before shader based pipelines knows exactly what it is, because you would have used the APIs by standards organizations themselves.
With ImGui you can definitely have a GUI that is:
1. Very complex
2. Able to use custom, complex widgets
3. High performance
4. Written very quickly with a very simple API
There are a few serious downsides as well:
1. It is not remotely conforming to the platform's standard UI (a bit less a problem for a DAW since they all have custom GUIs and widgets anyway)
2. The font rendering is done by the GPU using cached textures, so it's more limited in terms of what you can achieve there (not so much a problem for DAWs because you don't have much text to display)
3. Power usage can be high unless you use tricks to adjust the frame rate dynamically
4. It's in C++, which can be a con for some people (although it's written in old-school C-like style)
People do crazy stuff with Dear ImGui: https://github.com/ocornut/imgui/issues/5886
(And keep in mind that many of those are week-end / hobby projects, by people who prefer to focus on other things than the GUI of their projects.)