Quite a few anecdotes here validating the streak so I'll just be an alternate data point. When I used Duolingo regularly it was because there was a goal I intended to learn a foreign language for. The gamifed streaks never really had an effect on me and are quite off-putting.

I understand because of that I am not their target demographic. Or maybe I am and they want to engage users like me more. Maybe if the UI felt a bit more immersive--a clip of native French speakers interacting rather than cartoon bubbles and gems, for instance.