Also, I don't know about the semantic markup bit. "Semantic" markup (whatever that is) was the idea already when CSS was introduced. Now we need Shadow DOM, ie. additional markup, to make CSS manageable, which seems odd to me.
The "semantics" of HTML is really just that it's a structured document format with paragraphs, headers, lists, links, and a couple other idioms with a more or less tightly controlled inner structure. Next to these, custom elements seem out of place, and more like a general mechanism for page composition.
As such, they're bound to evolve over time. For example, for web components it's not difficult to imagine that a "container" component (for aggregation of other components, say a tab control as a trivial example) would be a useful thing to have. But then this calls for a way to place web components into the child content of other content, something which I understand isn't possible right now. Basically, we're looking into a long journey to morph HTML into something else here.