The criteria isn't scenes per page, its new mental picture per minute of reading time.

Don't see how that affects my examples. Here's another: how could a book of haiku have a less favorable ratio of 'new mental picture per minute of reading time' than a Dan Brown novel?

Then the ideal cultural production will speak to the times and touch on universal themes. These might be rare but will only be possible if cultural production continues indefinitely. Aside from these works one would want to consume an equal of "speaking to the times" works and "universal" works (holding constant for preferring one over the other generally). Unless we value universal themes a heck of a lot more than timeliness this means there is additional need for new cultural production even when that production doesn't speak to universal themes.

This is your best point so far. Now, diminishing returns doesn't mean no returns, nor does it necessarily implie converging on any constant (if I remember my limits correctly); but given a finite lifespan, hitting any diminishing returns means a suboptimal set of choices. So we could have thousands of Shakespeares waiting for readers, but if they are all eternal-veritied out, it's still a suboptimal situation.

This definitely blunts my argument. I think I can save it by permitting a small level of current-events production (if you produce too much, then it can't be consumed while current, after all), and there would still a lot of cost-savings - I saw my little sister with a copy of the very popular Pride and Prejudice and Zombies, which is certainly a current-events literary production if ever there was one, yet I'm sure it cost very little to write (Grahame-Smith claims he wrote only 15% of the final text, and the constraints surely made it much easier to write that), and no doubt much less than subsidizing universities to educate in creative writing hundreds of students.

But the people who do this aren't reading the Bible as literature (are there non-theists who just love the Pentateuch? Is the Koran any atheist's "favorite book" on Facebook?).

I'm an atheist, but I'll freely admit I derive tons of pleasure from the Book of Job, to just name one book. And as for the Koran: I was reasonably impressed on my read-through of the translation by its literary qualities, and I have been given to understand that the original Arabic was so highly regarded even by non-believers that Arabic literature can be divided into pre- and post- periods, and has since dominated Arabic prosody. Here's a random quick description:

"Besides making a masterful use of language on the level of words and phrases, it contains figures of speech, satire, and irony; employs a variety of narrative and dramatic techniques; and presents characters that, is spite of the sparse personal detail provided about them, come across as vivid figures. For those who can read the Qur'ān in Arabic, the all-pervading rhythm which, in conjunction with the sustained use of what may be called rhymed prose, creates in many sūrahs a spellbinding effect that is impossible to reproduce. There is the characteristic terseness of the Qur'ānic language which makes for some complex constructions, but which is difficult to convey in English without being awkward. The existing translations of the Qur'ān impose a further limitation, for they fall so far short of the highly nuanced original that a detailed study of the Qur'ānic language and style on their basis is well-nigh impossible." http://www.islamic-awareness.org/Quran/Q_Studies/Mirliter.html

(As for Facebook - if you're here, you can construct the social signaling argument why an atheist would specifically avoid publicizing his appreciation of religious literature, if he can even get past his own hangups in the first place.)

You would rather have us clumsily interpreting Pride and Prejudice so that is seems more relevant to promiscuous, polyamorous culture than just write new books?

We do that already, very inefficiently, via universities. And see my previous comment on Pride and Prejudice and Zombies... Writing new books is risky, as Jane Austens are rare; critics & interpreters, on the other hand, are plentiful & cheap.