I think that romance would have to be a necessity for future books in the series to keep its realness (plus, I'm a naturally sappy person). I do agree with all of you that too much romance in book five wouldn't be very good; after all, they are only fifteen! But I wouldn't be surprised if there was a little more sap in the sixth or seventh book. (PS ... I have yet to read OotP ... I'm getting it on Monday!)
On the topic of what would make me stop reading the books:
When Harry dies. ^_^ I'm dead serious though (no pun intended)... I love the books so much, the only thing that would make me quit reading them is if Harry died! Though I doubt he will till the seventh book, as the whole series is called the "Harry Potter" series!
Even then, though, I would most likely read every book over and over again, you know, just to keep Harry alive in my head! (I've already read books 1-3 13 times, and GoF 12 times 8)).