@rxfan Well, sure, there are stories with bad endings. But stories that are purely centered around a single relation between 2 characters, usually don't go that route, because people's sole interest and investment are in this specific relation, and if that doesn't go well, then the whole story just loses all its point (unless if to begin with it was an angst story, and not just romance). It's not like a different story with multiple relations, where even if a single relation is ruined, it can just be a contribution to plot progression for another plot element, as the entire plot doesn't depend on just a single relation.
The only case I can see it going south, is if the author decided to do a complete 180 on the plot itself as a twist, and go for a completely different kind of story. Like you would see in some genre deconstruction, or genre switch stories.