Let’s get this out of the way: The Dodgers have a closer… his name is Kenley Jansen. He had an off-year in 2019 and the Dodger coaches, as well as Kenley himself, believe he will return to form in 2020. It’s not like this has never happened to other relievers before. Even the greatest ones (well maybe except Mariano Rivera) had off-years.
The great Trevor Hoffman had many up and down years. In 1993, he had a 4.31 ERA and followed it up with a 2.57 ERA the next year. Then the next four years his ERAs were 3.