I think you're giving Duolingo too much credit.

Their lessons aren't bad because they want to stop you from being proficient in the language; they're just uninspired and unchallenging. Their gamification is nonsense and totally non-addictive. No one is addicted to Duolingo, otherwise they'd be doing hours of lessons every day.

People just don't want to break their streak - that's the reason they continue to use it. It's an obligatory thing you do once a day, it takes 2 minutes, and they get to show you an ad.

I've used it for a couple years learning Spanish, essentially because it introduces me to new words I'm otherwise not encountering in my regular Spanish usage, and that's all I need it for. Duolingo actually used to be better, and I was paying for it for a couple years. But they did a giant AI overhaul last year that made the content worse overnight. The stories are regularly nonsense because they're LLM-generated and seemingly not vetted properly. And they somehow even broke the TTS which hasn't been able to say certain consonant sounds for months now. But I digress.