You're right. Max Jones is an attractive piece and the value isn't off per say.
I was more speaking to people who talk out of both sides of their mouths. On one side they say "Hartnell is going to sharply decline." On the other, they say "you should trade him to us."
If a person believes he is going to sharply decline, they shouldn't want to acquire him. If they want to acquire him, that means they think he isn't going to sharply decline (at least for a while).
Columbus looked into moving him. But they didn't. Either nobody wants him or nobody offered the amount equal to how much Columbus values him.
Personally, I think Hartnell has value. He is a veteran leader who thrives and surrounds himself with young players. That has value, as you recognize. Also, his play has transitioned away from skating. Rather, he is putting himself in good positions. So he has a bit of a buffer before a decreased ability to skate will strongly affect his production.
I think it is smart to move Hartnell at the TDL if Columbus is out of the playoffs. That allows other teams to see if his production is still there, gives Columbus some contract/salary room, and gives Hartnell a chance to play on a contender.