Technically; yes, selling now would be smart. He's an valued player, regardless of whatever criticisms you can throw on top of him. Listen, he's an consummate teammate. He's going to be the silent, but hard working glue guy on offense, and particularly defense, that's in due part to his high and intelligent basketball IQ.
You're seeing the trend where big men that cannot be two way players, or cannot space the floor, and post moves big are pretty much the dinosaurs of the NBA. Here and there, you'll get excellent players that can score down low, but a common result of that has led to big men having to stretch the floor, and become more perimeter-oriented.
Horford can set excellent ball screens, and use his savvy post moves to free himself up, and create for others; an extremely underrated aspect of his game.
He's content with setting up the offense, and occasionally scoring in bunches, and as you can see a few games ago, absolute bollocks sometimes.
Like Draymond Green, Horford's number doesn't jump out the gate, but he's become impressive in how he has adapted, and fit himself ideally in Steven's offense. There is just no selfishness on this team, and Horford is due in part of some credit for that. The Spurs would kill to have someone like Horford, just don't say anything if we lose Horford, and he just goes off for 25/10/10 on us.