When a team is in a rebuild mode as the Braves certainly are, the last piece to be added when the team is ready to win again is a high dollar closer if one can't be developed internally.
Kimbrel is at the top of his game and had the most value he will probably ever have. Unfortunately for the Braves, as much as they needed to add young pieces, they needed to dump the Upton contract. Therefore, they added by subtraction, freeing up money and getting rid of a player that for whatever reason is never going to be the player people first thought he was. He has declined in virtually every way over the last SEVERAL seasons. They also picked up a few lesser pieces in the trade.
While I can see Braves' fans being disappointed because Kimbrel is the best at what he does, I honestly feel closers as a species are somewhat overrated and even a good one is good enough for a winning team and a great one is just a waste on a five hundred ball club as the Braves are likely to be for a few years.