Earlier quoted context omitted.
WebGL allows you to build completely custom UIs, and these are well suited for 3D apps (which typically use OpenGL on the desktop, so transferring knowledge is trivial). However, between custom OpenGL UIs and HTML-based UIs, there is an entire spectrum of native apps (mobile and desktop) that make use of platform conventions and standard functionality. This space is difficult to cover with web technologies because yo…
Now you mentioned it also passed my mind to scrap DOM and use my own gui, lots of advantages but im worried it will slow down the development. Indeed all the timeline editor is a canvas. Its worthy to consider it.
That said you seem to be far enough along that redoing the GUI would not be a good idea really.
Also the DOM is a fairly okay retained mode UI, so if that is your preference I would just use it.