> Even if it's a success, the likes of DuoLingo can copy that in a month and have so much more marketing and money to win the market

Only theoretically; in practice, Duolingo have a specific brand — IMO aimed at helping younger kids with schoolwork — they don't want to risk damaging, so if you target your language course at people who chafe at the style of art, animation, voice, and/or content of Duolingo[0], you still have a business opportunity even though Duolingo is also already working with GPT-4.

[0] This is why I've stopped using Duolingo entirely, while also having paid versions of Clozemaster and Babbel, which are also very different from each other.