I think you've misunderstood my point; I don't think that at all.
I'm specifically criticising the focus polymer has on creating user interaction elements.
My point is that this sort of low level functionality isn't the right place to focus; most low level components you need already exist. What people is a way to build robust application UIs (and yeah, that includes having a root level <App/> component).
For comparison, read:
https://www.polymer-project.org/2.0/start/
'build custom ui widgets' <-- wrong, this isn't a problem most people have.
vs.
https://vuejs.org/v2/guide/#Composing-with-Components
'build your application from components' <-- right.