Let me first say again that I do agree that our team is better with Kimbrel, and that it's not just in the closer's role, but that it lengthens the bullpen to the 7th inning. I have no issue with having Kimbrel on our team. I am happy with that.
As far as Kimbrel's decline goes, there has been a slight downward trend since 2012. It is not one that I am overly concerned with, but it does exist.
Here are some of his stats from 2012 to 2015:
ERA: 1.01, 1.21, 1.61, 2.58
FIP: 0.78, 1.93, 1.83, 2.68
WAR: 3.3, 2.2, 2.2, 1.5
K%: 50.2, 38.0, 38.9, 36.4
BB%: 6.1, 7.8, 10.7, 9.2
HR/9: .43, .54, .29, .91
BAA: .126, .166, .139, .185
WHIP: .65, .88, .91, 1.04