It's not the pairing of the romantic leads that's the problem: the problem is that, with a lot of these shows, they arrive at a point where they find themselves asking, "Okay, we paired the leads together... Now what?" And it becomes clear that, for many of these shows, there was never supposed to be a "Now what?" They solve the 'unsolvable mystery' and/or bring the Big Bad to justice, and then the two leads get together... and then, that's supposed to be the end of the show. But what happens is that a lot of these shows get so popular, that the networks make the actors and the show-runners Godfather Offers to keep them on the air, and throw so much money at them, that they can't make themselves turn it down. So then you arrive at the point where the writers have told all the stories that they actually wanted to tell, but now they're under contract to write fifty more stories. So, what happens next? You go to the Big Book of Tropes, that's what happens. And that's when you end up with all of the re-tread plots that you hate so much.