Canvas is immediate mode rendered and already breaks through the DOM. There are also popular movements to make the DOM appear more as an immediate mode abstraction (React). In contrast, most retained mode 3D abstractions are seen as pretty bad.
Everyone thinks that they can do better with an immediate mode API, then we start building some kind of data structure to track down what needs to be drawn and when, with the result being similar to the traditional joke of half-implemented Lisp or ORMs, but applied to retained mode rendering.
Sure, some experts manage to get it right, and those are the ones that get to write AAA game engines, but many don't.
I can't think of a retained mode API that works as well for RTS games as it does for FPSs, to say nothing of non-game uses like CAD.
... but they are not built on an abstract retained mode API. That would be the wrong level of abstraction for a Web API that people can build on properly. I think that's the point here.
Unfortunately, Web APIs tend to be far too high level while missing out on low-level hooks, like the disaster that is Web Audio (and media playback in general).