The way I see it, the game should not be compared so much to a car (which is essentially a tool), but rather to a piece of music, or a book, or a sculpture, or a painting. Suppose an author published a book and then decided to add a new chapter! I understand this falls into the category we all agree makes the game incomplete. It's like those DLCs that add epilogues. My position is that even if the writer added one single paragraph, say, a description of a place, or one dialogue (that might not be essential for the story to make sense)... all that would still render the original publication incomplete, because those details are fleshing out the characters, or the world, or even just the tone of the narrative.
I think it all comes down to whether we think of games as an entertainment service, or as artworks. I tend to put them in the latter category, but I concede that there are games which are clearly just about spending some fun time, and not really about communicating anything (for example sport simulation games).
As you may imagine, I am much more interested in games which are about exploring some world, or some story, or a set of ideas, than those which I classified as "entertainment games". In any case, I can't avoid feeling like a game is incomplete when there's "extra" content that's missing.