I think Qt is only missing well-written, feature-complete bindings for a major JS runtime, including support for hot reload.

Developing UIs without hot reloading is too painful.

I think what you're asking for has existed for a long, long time. QML.

QML doesn't have a way to define interfaces with JSX and doesn't integrate with the wider JS tooling. From my very limited experience, it still feels too close to the C++ world.