This is where reality trumps perception.
In September, he has made 10 appearances. He has 6 saves in 7 attempts (Wednesday night was a save attempt in my mind, but since he was taken out, he technically didn't get a blown save). For what it's worth (admittedly not much for a reliever), his ERA at the start of September was 2.86. Going into Wednesday night, it had gone down to 2.65. If one assumes that the the unearned run he allowed in Oakland on the Holt error as earned, his ERA rises to all the way to 2.82 before Wednesday). In his 9 appearances up to Wednesday, he was perfect 4 times and allowed 1 runner 4 other times (1 H (albeit a home run), 2 BB, 1 HBP).
While Britton seemingly hasn't been scored on since Yaz was a rookie, this month in 11 appearances he has allowed (gasp) 10 runners in 11 innings (he's been perfect 4 times).
Do I wish Kimbrel would be more efficient? Absolutely. But looking at his record, he has always walked his share of batters (not as badly as this year I will grant you). And I think that knee has something to do with it. If a reliever is going to allow 6 runs in a month, I would rather it be the way Kimbrel has done it (1, 0, 0, 0, 0, 0, 0, 0, 1, 4) than something like 1, 0, 1, 0, 1, 0, 1,, 0, 1, 1.
As for Koji closing, do we know if his shoulder would stand up to back to back or back to back to back appearances? I doubt it.