I've done a bit of research in the meantime and looked up definitions of literature in different places. Turns out that the mess that is this thread is a direct result of the mess that is the definition of literature in general.
Originally just a term for anything written it later got that quality twist that is reflected in a definition like
Merriam-Webster's: "written works (such as poems, plays, and novels) that are considered to be very good and to have lasting importance." I think many people operate with that definition, and probably had it thrown at them in school.
The definition I have put forward earlier derives from the Russian Formalists and their
literariness: "The subject of literary science is not literature, but literariness, i.e. that which makes a given work a literary work." (Roman Jakobson, 1919, quoted from
The Penguin Dict. of Literary Terms and Literary Theory, 3rd ed.). According to the dictionary just quoted, literariness is closely connected to
defamiliarization as defined by Viktor Shklovsky: "The technique of art is to make objects 'unfamiliar', to make forms difficult, to increase the difficulty of length and perception, because the process of perception is an aesthetic end in itself and must be prolonged.
Art is a way of experiencing the artfulness of an object; the object is not important."
So in the end there is not really a basis for a decision, because everybody will abide by their own definition of literature. I think the emotionality of debates like this shows how deeply engrained the distinction between good and bad literature is, even in people who say they don't care about it. Blame the schools.