In an era when relievers throwing in the upper 90s is more requirement than luxury, ace relievers seem to rise and fall with incredible frequency.
Two years ago, the best seven relievers by fWAR were Kenley Jansen, Craig Kimbrel, Roberto Osuna, Corey Knebel, Pat Neshek, Chad Green and Andrew Miller. Just a season and a quarter later, Jansen and Osuna have been good but not great, Kimbrel can't find a job, Knebel is out with an injury and Neshek, Green and Miller all have been below replacement level.