For most viewers, the surest sign that a show's going downhill is when the main character is suddenly written off. Nobody thinks the golden era of "The Office" includes those two seasons where Michael Scott was off in Colorado, just as nobody's favorite season of "That '70s Show" is the one where Eric's off in Africa while Seth Meyers' brother swoops in to date Donna in his place. Even when a show manages to replace its lead in a surprisingly effective, fan-pleasing away — see "Community" season 5 — there's still that pervasive feeling that the show's on borrowed time and its writers are delaying the inevitable by trying to keep the party going.

In 2019, "Shameless" became another in a long list of TV shows to have its main character leave prematurely. Emmy Rossum's Fiona Gallagher, the oldest sister in the family and the one who served as the other Gallaghers' stand-in...