The fact that the new article and section elements can now have their own header hierarchy doesn't mean we should "use only one heading element (h1...)". The beauty isn't that we don't have to think about the hierarchy anymore but that now each distinct "document" on the page can have it's own h1-hn hierarchy.
Sure, but why would you still want to use the other hn elements? The only reason I see is for subheadings within a hgroup, but I'm not a big fan of that tag.
Well, look at your own article. There's an h2 tag wrapped around each of the subheadings (e.g. "NO MORE H2-H5").
How do you propose content authors mark these up if they don't use an hn tag?
The problem is that the subheading in my articles is really part of the main heading. Without it the heading itself loses meaning. When using the hgroup tag only the main title is considered for the document outline so I'm losing valuable information there. As for the fix, I'd rather split both titles in the h1 tag itself. That way both titles remain 1 whole, even for the document outline.