You're just further illustrating my point. Henry was holding us back in his final season too. The guy was broken and when he was playing he was moody and played with low energy. Didn't Wenger himself say Henry leaving allowed others to play with more freedom?
If a key player is only fully fit for 1/4 of each season, he becomes more of a problem than a blessing. That why when they leave the world doesn't fall apart and often it usually gets better because you're finally able to address the issue.