What do you mean by "component based declarations of UI elements"? Declarative specification of UI widgets (e.g. XUL) is much older than web-based component frameworks like React. Component orientation in UI frameworks is even more traditional, and far older than Qt.

What "recent ideas" do you find in Jetpack Composer, Flutter and React Native that Qt lacks? These tools let you write apps with a nice user interface for Android exactly like Qt; supporting Kotlin, Dart, Javascript etc. besides C++ and various Qt language bindings is useful, but not necessarily superior.