Time for a crackpot theory!
In many ways, the Dominion War was a godsend for the Federation/Klingon alliance. The Klingons, steeped as they are in warrior nobility and all that rot, sent their finest warriors, the angriest, the ones with the greatest bloodlust, to the frontlines.
And like the fields of Flanders, the battlefields of the Dominion War were meat grinders. The Klingons bled themselves white.
The most hostile, the angriest Klingons, there were the ones who took the brunt of the fatalities. Not the poets. Not the philosophers. The warriors.
It’s evolution in action. The genetic predisposition toward violence and brutality took a hit in the Dominion War. Perhaps a massive one.
Who knows? In a few generations, we might see docile Klingons.