Duolingo isn’t a good resource for learning a language, it’s focus is user retention

These two statements contradict each other. To learn a language you must practice it every day, week after week, month after month. It's an appropriate application of addictive game mechanics, because our motivation doesn't last long: 1-3 months for most people.

Duolingo might not be the best place to learn some languages (e.g. German), but it can be a very helpful tool for everyday practice. And stuff like streaks, leagues, and other things are rather helpful.