I agree with your point, yet as the parent points out. The uptake will be less, it has to be. Websites and people learning languages are vastly different size wise.

I also think the numbers he quoted about buying learning material are a little off. Purely because they are quoting numbers which cover a broad range of media. Whereas duolingo is the web and maybe mobile apps.

I personally think the market is huge, I just felt the numbers were sexed up for effect.

The "avg" of user sentences to create a better version sounds very exciting from a tech point of view.