The problem with all these alternatives is that the language selection is extremely limited. You want to learn English, French, German, or Spanish? Great, there are a million options for you! But if you go a bit more niche like Finnish or Irish, your options are much more limited. Of course there are ways to learn those languages - and much better ways than Duolingo. But Duolingo’s strength is offering a bunch of them, for free, in one place.

Note that I’m not trying to defend Duolingo, but rather deploring the lack of alternatives.