[bluGill]

The problem is progress is slow and hard to measure. Streaks are something you can measure and consistent study makes a different after many hours of work.

That said, if you want to learn a language you need to study a lot. Complete everything Duolingo can in a few months and then find better courses of study to learn after that. You should never in my opinion have more than a 6 month streak.

[lupire]

How is "used app every day" better than "did X exercises per week" or per month?

[bluGill]

Better for who, you or Duolingo? If you want to learn a language you need to run out of Duolingo exercises in a couple months and quit that app for other study.