Early on, the Duolingo stated goals was to teach language to the point where it's learners could ultimately start translating documents. They were going to sell cheap mechanical turk style translation services. (Think captcha style translation)

Unfortunately as they got popular automated translation services got good enough that nobody was going to pay for a slightly better and slower translation enmass.

Once that happened, that's when it seemed like they dumped their goals of teaching language and instead focused on dark pattern money extraction.