I figured you'd be into the romance by then, and could opt to keep it going or end it. Most DA romances have a similar point.
That could work. I'd kind of rather Varric made the choice to stay with you, though, since I'm picturing a conversation of, "Well, Inquisitor, should I go back to my girlfriend?" and I can't see any way to do it that wouldn't just be horribly awkward. But yeah, maybe if Bianca showed up late in the game, and he decided to break it off with her dependent on whether he was with you at the time? Or something? I dunno, I'll admit it still kind of sets my teeth on edge.