Not sure if I gave off the wrong impression, but I am not a proponent of having only sum types and discarding hierarchies; as you well mentioned each is well-suited to specific usecases.

I do however disagree on subclassing being the best fit for UI code. "The Elm architecture" as well as the model presented by React functional components with hooks offer sets of tradeoffs that I at least have found are better for most of the development I find myself doing when writing (and particularly when maintaining) regular business frontends.