The clergymen of Mather's day wanted a stable, predictable social structure. Their claim to authority depended on it. But the world was changing so fast that they couldn't control it. Since the ministers were most frightened about the uncontrollable future, they bemoaned the fate of the future generation and blamed the schools. The pace of change has not let up since 1690.
Schools keep changing too.
By some measures they keep improving. But change still breeds anxiety, and a good jeremiad still sells. It's no wonder. You get to affirm the values you prefer and share them with lots of other frightened people. The more people bemoan the sad state of the world, the better it feels.