Why shouldn't your entire application be a widget? Sometimes it's nice to embed one application into another, multiple times. I find your reasoning here short-sighted, since there's often no clear point at which components should bottom out. You can always add another layer.
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).
Actually - the process of creating both is exactly the same, I created components in angular, react and polymer. No difference IMO (apart implementation details).
That "sometimes" quickly breaks down when your application starts to grow beyond trivial. Some things that are rather application-specific like routing and dependency management don't quite compose well in the widget model. Or at least, Polymer at the moment doesn't have proper solutions to quite some edge cases that you'll be running into.
I actually implemented React's UI Router in polymer, works the same as in Angular 1/2. The biggest issue with web components in general are problems with Shadow DOM support.
Which means you need to do elements without shadow components up to top level - that is indeed annoying and reduces the pros of components.
