Worth keeping in mind - every Pixar film released in recent years, including the just-released Lightyear, began development and production under Iger's watch. Pixar's significant decline started only a few short years after the aquisition, really only once they started releasing projects Iger had a hand in. If anyone is responsible, it's the man who bought it.
I don't really say that in defense of Chapek, but it should be remembered that their 4 projects released under his tenture were pretty far along by the time he showed up. We haven't really seen his touch on their movies yet.