You're right - my argument is intended for easily reachable improvements.

Your brain is full of neurons that are sending chemical messages to one another. For this to work, the brain has to be able to receive all the messages sent, and to send all the messages received. Your extra chemicals have to change this process somehow, and they can only do this by interfering with the signalling process. But your body already knows how to make these signals.

You're not creating hard-to-reach improvements by adding chemicals. You can only alter the balance of the chemical network that already exists, and evolution has already balanced that network pretty well.

If you could do something more radical, like adding a different kind of signalling, or fundamentally speeding the thing up, then I'd be interested. But chemicals can do neither of those things.