Of course I understand why Horford's salary is so large, but that still doesn't change the fact that he has the 11th highest salary in the league and is no where near the 11th best player in the league. He is 24% of Boston's cap space this year and based on estimates of next year's cap will be close to 27%, and something like 25% in his final year (if he picks up the option). Horford is a very good player, but he isn't worth that contract.
You are begging for a false premise: That league salaries should be ordered in accordance to talent.
They are not. They are not every going to be. They are not really even supposed to be.
Whether a player is "worth" a contract is not based on this sort of dubious and contrived ranking. A player on a rookie contract or a value deal blows that sort of methodology completely away.
Here is the proper way to assess whether a player is 'worth his contract'.
The job of a player is to help his team win. That is what you are paying him to do. His value is realized in how well he does that and can be estimated by estimating the share of his team's wins his contributions resulted in. There are various analytics that estimate this, one of the more commonly known being BBREF Win Shares.
A win has a dollar value. Based on BRI projections, I'm going to conservatively estimate that a Win Share this season will probably be worth on the order of $4M - $5M.
Horford is on pace to generate about 9 Win Shares this season. That would put his revenue value at somewhere around $40M, plus or minus some handful of millions, but probably well, well above his salary.
Note, due to the size of Boston's market and the fact that wins on this team will contribute to a playoff run (thus generating playoff revenue), the case can be made that a Win Share is more valuable to a team like Boston than it is to the league on average. Thus the real number is probably higher.