> a single React/ShadCN button

So don't use ShadCN? It's so weird to put up this strawman app and then be like "see what's wrong with React"? Like showing two boards nailed together and being like "can you believe I needed all those power tools just to do this?"

> Add more widgets—tabs, modals, whatever—and that gap only widens

This is the benchmark I want to see. Two full-featured apps built with minimal prod dependencies. There's a pretty good chance that the various ShadCN modules share many of their dependencies so that importing more doesn't necessarily mean a linear increase in bundle size. It could be that once you build something full-featured, React projects come in smaller, or at least not big enough to invalidate the other upsides of choosing it.