In the end child geniuses could grow up to be average people.
People who start walking early don’t walk better than the rest of us as adults. People who are toilet trained early don’t use the toilet better than the rest of us adults.The idea of some sort of language head start making you mentally superior is nonsense. There’s no causation showing that babies who are only spoken to in proper English will grow into geniuses with bigger vocabular.
Kids get less and less time to be kids, as it is. Now we’re going to get uptight about THIS? What matters most for hearthy brain and social development is enough attention and stimulatio. Baby talk is rarely pure babbling and it often qualifies as a form of communication, or at least modeling it, anyway. You’re conveying that there are words for things. That’s more important than the baby knowing which words mean what. Have you ever met an adult who spoke baby talk to you because they don’t know better?