He isn't the 11th best player in the league, but he has the 11th highest salary. He is 31 and signed for huge dollars for 2 more years.
This again is engaging in the same kind of shallow sophistry as one can find in click-bait articles like the one in the OP.
You know very well that many of the best, most valuable players in the league have their salaries artificially limited by the CBA, i.e. rookie contracts and limits on max salary based on years played in the league. Others signed their current deals before taking their games up a few notches.
Horford, if anything, is one of the few star players in the league who is probably
appropriately paid for his contributions.
He's the second best player and the unquestioned veteran leader and culture-setter on a team that is going to win 50+ games and finish with a top 3 record in the conference, likely top 5 in the league.
The only reason that may not seem as though it's "worth" max salary is because the CBA causes so many players to be extremely underpaid.
Furthermore, I'd rather have Horford than Smart + Monroe, and the salary cap situation can only fairly be assessed by looking at all of the player salaries together. Al Horford's salary alone does not push the Celtics within shouting distance of the luxury tax.