(I started buying at $110 yesterday.)
I want to remark that DUOL's main asset is its brand image, with 5m/17m followers on instagram/tiktok. All the speculation about AI replacing justifies the decline only if AI actually makes users quit the app and turn to a personalized LLM tool (or something similar) for learning.
I think this logic is irrational. Do you really think the majority of people will seek the most productive way to learn, rather than feeling productive while having fun for keeping up their streaks amid the social hype? Duolingo knows this well, which is why they have introduced chess and more.
A serious drop in quality would have to occur for that replacement to happen, but the company will likely try to maintain quality using all possible tools, including LLMs. I understand the increasing complaints about quality, but I find it hard to believe it is becoming terrible. Personally, I am studying at a lower level (B1) of a major language (French), so maybe that's why I'm satisfied. I definitely think there is room for improvement here and there, but no competitor can achieve everything.
(I used gemini just for the grammar polish)