I didn’t watch the video but his defense and QB situation led to his downfall. His DL was badly aging and they couldn’t stop anyone. The drafts began to go sour and a number of the misses were on defense.
Landry’s innovative defense was the flex defense in the 60s and he stuck with it into the 80s and it caught up with him. Even his players tried to get him to move away from it. When he finally did it was too late.