I really like the ENT episode "Judgement" because it opens up a dialogue about other careers in the Empire being viewed as less honorable than choosing to become a warrior.
Here is a short dialogue between Kolos and Archer
"You didn't believe all Klingons were soldiers?"
"I guess I did."
"My father was a teacher. My mother, a biologist at the university. They encouraged me to take up the law. Now, all young people want to do is to take up weapons as soon as they can hold them. They're told there is honor in victory – any victory. What honor is there in a victory over a weaker opponent? Had Duras destroyed that ship, he would have been lauded as a hero of the Empire for murdering helpless refugees. We were a great society, not so long ago. When honor was earned through integrity and acts of true courage, not senseless bloodshed."
"For thousands of years, my people had similar problems. We fought three world wars that almost destroyed us. Whole generations were nearly wiped out."
: "What changed?"
"A few courageous people began to realize... they could make a difference."
This only warriors are worthy of honor societal trend is the cause of Klingon decline I think. The Klingons of TOS never seemed preoccupied with honor. Not even the TOS films that overlapped with TNG shows. Like TUC and season 5 of TNG.