The idea of i + 1 makes a lot of sense for second language acquisition because it also jives with what we know about *first* language acquisition. Linguists, for instance, talk about "caretaker speech," which is not exactly baby talk - but caretakers talking to small children do tend to use simplified language that is just above the child's own current level.
The idea that output is not important seems less convincing. With children's first language development, research shows that it is important for adults communicating with the child to respond to the child's output. (If the child says "nana," for instance, you might say, "Do you want to eat a banana?")