"Humanity has declined" is the story of a remnant human population in a post-singular world, and their interactions with the New Humans, which are small fairy-like creatures with more or less incomprehensible motivations. This is a tale haphazardly told in the light-novel form. Humanity's decline is actually kind of a success, humanity no longer pollutes its environment, no longer kills animals. Humanity is not organised enough to engage in self-glorification (there's an ongoing plan to put up some sort of monument to humanity, which never goes anywhere). Pioneer and Voyager have been returned to earth, the aspiration they represented has no meaning now.
"Gargantia on the verdurous planet" is the story of a soldier from an endless galactic war who finds himself stranded on a backwater planet in the company of a more primitive people. Again we have humanity in declined form, here living on a large oceangoing habitat. Similar to the island in Greg Egan's "Distress" and the floating city in China Miéville's "The Scar", this is an anarcholibertariosocialist utopia in physical form. Again we encounter humanity that has declined and yet advanced or even advanced by declining.
I'm not aware of any precedents for this in Japanese fiction, as far as I know it's new.