> Qt might have at least led to some good results

You seem to focus on the outcome while what interests me is the programming model. Say what you want about object-oriented programming, the problem-space lends itself very well to it. In Qt/Swing/… components are just classes whose behaviour and appearance are determined by function overrides, taking as parameter event or graphics objects. Just like that, you cleanly solve everything that "webcomponents" stands for: interface contracts are determined by class constructors/parameters, customisation is just class inheritance with member override, look&feel customisation is plain calls into the graphics API primitives, layouting/responsive-design is taken care of at the just level of abstraction, and all of it takes place in the same language and ecosystem/stdlib (no HTML to CSS to JS references to keep in sync and mental overhead).

React reproduces some of this model, but with much more technical and mental complexity, bringing trade-offs of its own. I'm not calling this easier to work with, simpler, or easier to debug. It certainly gives more creative options, but only because the ecosystem is broader (and full of supply-chain risks).