I’ll start with the Cliff Notes version of my argument. (1) CC Sabathia is no longer a big-league starter. (2) The Yankees have three southpaw relievers, but no true lefty specialist. (3) CC’s decline has been entirely against righties, so he still has use as a lefty specialist reliver. (4) This isn’t as weird as it sounds: other declining lefty starters have had second lives in their 30s as relievers.
I’ll skip the Nth discussion of how CC has been horrible for about 300 innings spanning 2013-2015, except to add that CC has zero value to the club even if he could run a 4.