Sometimes good series not only go bad, they do so at the worst of all possible times — as in, right when they’re coming to an end. As a result, instead of leaving us reflecting fondly on a show that we’ve loved and lost, we’re stuck with an unpleasant aftertaste and a nagging question: It wasn’t always this crummy, was it? The answer, thank the TV gods, is generally no, our favorite programs didn’t always suck. But that doesn’t diminish the “Ugh!” of a lousy series finale. And with so many shows signing off recently — everything from Shadowhunters to...