This. A thousand times this.Also, I don't think top-notch closers are properly valued by WAR, but that is a different and lengthy discussion.
I'd be comfortable betting the house that the Sox don't win the division last year without Kimbrel. I might even bet that they don't make the playoffs without him. Even using Fangraphs WAR, which rated him higher, he was only worth 1.2 wins. They won the division by 4 games (and the Wild Card by 5), so you might say that Kimbrel only accounted for a little more than a quarter of that difference.
However, WAR does not effectively account for the trickledown effect we would have seen if Koji was a our closer last year when all of the injuries in the pen happened. Or the impact on the rotation if they were even more hampered in their ability to pull struggling/tiring starters.
Pull Kimbrel from the pen and replace him with the mythical replacement level player, and at the very least I think they are a WC team instead of a division winner, and I wouldn't be surprised in the least if they had ended up 5 or 6 games worse than they actually finished. Maybe more.