You're generalizing in just the same way that the GP is. Whether learning to read and write is a chore or an enjoyable pursuit at that age varies from kid to k

You're generalizing in just the same way that the GP is. Whether learning to read and write is a chore or an enjoyable pursuit at that age varies from kid to kid. If it's an enjoyable pursuit, that's great. If it's a chore, then forcing that kid to do it because of some arbitrary standard applied to all kids might end up being a net negative. The gains from being able to read and write earlier might be outweighed by increased negative feelings associated with learning and teachers and school. As the OP said, this can cause students to become disengaged and actually fall behind others with whom a more patient approach was taken. It's basically a form of burnout, before the poor kids even reach double digits.