Kimbrel has gotten the job done in save situations, he's 21 out of 23 chances (plus the "hold" he was given in the 4 walk game last week, he didn't blow the save but he did pitch his way out of it.) He also is unscored on in 33/40 appearances. When Kimbrel blows up, he blows up spectacularly, but his issues have generally come in tied games and games where the Sox are behind. Given his psyche, I don't see putting him in a set-up role as working very well. And I acknowledge he's had some high wire acts, but overall he's gotten the job done. Not to mention he's got close to 250 major league saves over the past 6-7 years. Even Mariano Rivera, the GOAT, would blow a few saves a year.
Barnes has been spectacular at times and decidedly mediocre at others. Yeah, he's 1/1 in saves and has 10 holds, but he's been scored on in 15 out of 46 appearances and only lately has begun making regular appearances in the 8th inning.
Now is not the time to experiment.