What your heuristic may be accurate in the general sense, it's absolutely not the case for ImGUIs. I've used the technology on a real commercial product before

What your heuristic may be accurate in the general sense, it's absolutely not the case for ImGUIs. I've used the technology on a real commercial product before and it was vastly superior to the old way of doing things. The "BS" complexity of retained-mode GUIs, as you suggested, is not really BS but the logical result of using a client/server model. That said, for the average desktop application (where all necessary state is local to the user's own machine) this model really is a poor fit and ImGUIs shine. A lot of the evangelism comes from the fact that many more conservative devs still insist on using the more complex client/server retained-mode paradigm, even when immediate-mode is more suitable to the task at hand.