The thing is, the input hypothesis all by itself is not enough. It's arguably close to where the modern paradigm of second language acquisition started, but it's not where we still are nearly 50 years later.
For example, one big thing that Duolingo's method completely misses out on is the importance of a rich communicative context. This was implicitly there in Krashen's original monitor model, but wasn't fully appreciated until closer to the turn of the century.
For example, one big thing that Duolingo's method completely misses out on is the importance of a rich communicative context. This was implicitly there in Krashen's original monitor model, but wasn't fully appreciated until closer to the turn of the century.