Speaking as a total ignorant from a coding perspective. But I guess that wouldn't be the hard part, considering that most of Duolinguo is just boxes and text inputs. How difficult it is to create a database of competent linguists with an efficient training who can progressively enhance your understanding of languages?
Try taking a break for a month and see how much you actually remember. In my experience it was depressingly little, and I’m not generally bad with languages at all.
I think that would go for most learning methods. When you don’t practice a skill you’re always going to get worse with it over time, especially if it is a language.
After a certain point you should be able to retain a language for more than a month - after you’ve attained B2ish level I’ve heard.
What I am saying is, I don’t think people who use duolingo are any better/worse off than most other methods.
According to some guy on Youtube, that’s less of a learning method thing and more of a getting over a basic threshold of competency thing. I forget exactly which level he said it was, but the claim was that if you reach at least B2(?) you won’t forget it anymore.
I think this heavily depends on your learning type. For some it may work for others not. What is important that it actually helps some people and these people have no foss alternative around.