Great essay! I agree with you, about not needing to "sell" a ship in an established-relationship fic, and so on. However, there are other reasons beyond "needing to sell the ship" that an established relationship fic would have, as you put it, a mini "how they got together" fic early on, and why some readers might want that. I am as OTP as humanly possible on Sam/Dean from Supernatural, and I don't need to be convinced in any way. However, I have very little interest in established relationship fics -- those just aren't the stories that grab me -- and the only way I can stay interested in an established-relationship story is if I have some sense about how they actually got together. Otherwise, I feel kind of... cheated. Again, this is all just my preferences as a reader, but I did want to point out that "convincing non-shippers" isn't necessarily the reason for people wanting that, either as writers or readers, you know? As you said yourself, the first-time story is extremely popular; including that "how they got together" bit just as easily functions as a way of capturing that particular rush of energy, rather than as a way of convincing non-shippers.