You miss the last layer: user and browser. Even with all the improvement is web technologies in recent past, the browser is still not close to native widget.

You miss the last layer: user and browser. Even with all the improvement is web technologies in recent past, the browser is still not close to native widget. And finally the users themselves. No matter how neat you managed to be in the underlying layers, the UI is messy. It can change fast, meaning either massive cost rebuilding an entire application, or breaking those neat layer. Users also wants everything connected to everything. It does not matter if those connections are explicitely done via ugly spagetthi code or implicitely through clever abstractions, they effectively exist and that's how the requirements, user experience feedback, bugs and testing will be based on.