Why is the happy ending construct important and do you see that changing?
Sarah: I think I just answered this question! I don't see the core of two (or more) people committing to being happy with each other changing. What that core might look like (not marriage?) might change, but the core of romantically- and sexually-involved people agreeing to try to be together never will. And I don't think it'll be a bad thing if marriage is no longer the be-all and end-all of romance HEAs. I don't think it has to be, and I think the HEA can still be emotionally satisfying.
Eric: I think the happy ending gives shape to the rest of the narrative, cushions our reading of terrible stuff that happens en route–"we know that somehow, somehow, this will turn out OK, so we don't give up, just as the characters don't give up–"and for the author, it provides one of the givens of the genre, a set of reader-expectations which she can adopt or play with in various ways. (Just how far from that norm, for example, can she swing the tone of the final chapters? If there's no norm, there's no challenge; as Frost said about free verse, it's like playing tennis without the net.) I'd also echo Sarah about optimism here: romance is a genre that fosters and nurtures optimism, even faith, in its readers, and the happy ending is central to that. (I've joked in the past that “every romance novel is an inspirational romance,–? but that's not entirely a joke.) Now, as for the shape of the HEA, I think it might well change a little here and there, in the ways that Sarah describes. In fact, Pam Regis says that the betrothal–"the affirmation and formal pledge to commit to the relationship–"is the crucial element, and not the marriage per se. You can play that betrothal in a LOT of ways: explicitly, of course, but also implicitly, more delicately, through a gesture or allusion. (See, for example, the final paragraphs of the two romances I first loved: Bird's The Boyfriend School and Crusie's Crazy For You. No spoilers here.)