This is entirely a problem of AI sycophancy. You would expect a system that has expert level knowledge in so many domains to be overly cynical and straight up tell you that "No, there is no market for a language learning app, Duolingo doesn't teach language it uses dark patterns and fake testimonial to get people addicted"

AI is actually fine at telling you objective metrics for your field of interest, it's just that people ignore them because they already have the thing they want to do locked in their head. They want to build a skyscraper out of toothpicks and the AI does help them, but the goal remains elusive.

The most important metric in language learning is willingness to communicate, an app (duolingo or flashcard) that lowers WTC is the opposite of a language learning app.