Duolingo is useful, but not efficient. When people say I want to learn a language, they often mean I want to learn this language efficiently, e.g. to be able to write an essay like the post says after a realistic period of time.

I personally don't believe its pedagogical deficiency is mere incompetence. The whole business model is to keep you on the platform as long as possible, so why would they make you learn faster rather than just enough to keep you there?

As a long time user before, I have observed a lot of mechanism changes that bear out this observation.