You're a little too kind to Duolingo. It is useful for the very, very beginning, but people sink a ton of time into it which could've been used to actually learn the language.
Making something as fun to use as Duolingo but that actually teaches you the language is an open problem.
> but people sink a ton of time into it which could've been used to actually learn the language.
Or it would be used to do something completely different that is nor language learning at all. There is this hypothetical world where the 10min of duolingo before sleep with some binging here and there is the only thing to prevent you feo. regularly spending considerably more effort (and time) if a more serious effort.
That is just not how it works.
Here is the thing - Duolingo is actually teaching things. Slowly. And not things of your choice. But you are slowly progressing. And it gets you further then downloading anki deck or graded reader you find boring or even language transfer and giving up on them three weeks later.
You can make an app with different trade off or more fun app. But you will have to choose between causual and intensive.
Yeah, that's fair -- you can view Duolingo as just, basically, a fun game, and you do learn something.
But I do think there's space for something equally entertaining (not anki decks!) and more effective.
I learned Spanish decently well, and I think one of the most helpful things I did for that was just hanging out with people, speaking Spanish, and drinking -- not grueling at all, very fun!
Making something as fun to use as Duolingo but that actually teaches you the language is an open problem.