It isn't free -- you have to watch ads, which detracts severely from the learning and enjoyment.

Plus being a betrayal of their user-base which is heavily weighted towards young and impressionable learners. And, by the way, a betrayal of their initial promise, that they would never do advertising (expressly stated in the first promo video they released, which is still on YouTube).

What you have to lose is your time spend. At least you'll lose time due to the gamification and the ads.

Anki is useful for spaced repetition, but setting up a deck is going to take effort and requires knowledge (building a deck is useful by itself though).

https://tatoeba.org/eng/ is a partial solution to the problem that someone has to create the content.

You ever hear of a thing called opportunity cost? Some people interested in learning a foreign language actually have a compelling desire to learn said language.

> Creating such an alternative wouldn't only be very difficutl tech-wise

I am quite curious how you figure this. Other than analytics and tracking, Duolingo doesn’t even require a backend.