Duolingo does not need a moat. They are already an established incumbent and have market inertia on their side. They can afford to experiment and make mistakes, and then to backtrack those mistakes.
For what it's worth, I also stopped using Duolingo when the human forums closed. I often found as much value in the forums as in the actual course. And like GP, the hardest part was relinquishing my nearly 1000 day streak.
I'd love to see someone build Duolingo with a single system prompt. By HN standards, no one would be paying for Character.ai, Cursor, Windsurf, or dozens of others tools because they could just call the ChatGPT API themselves.
you cannot build a moat with LLMs, so there is no value in using any service that is wrapper around the chatpgt