There may be a very fine line between reward-hacking for the user's benefit vs. building a facsimile of language learning on top of a nuclear-powered Gacha loop. I've looked at other learning apps (including I think Anki but I haven't tried it) trying to help people out of the Duolingo pit and they do all seem much more clinical in comparison. Not automatically a bad thing for an educational tool but it's also not hard to see why they don't get the same traction.

yeah, the issue is that there really can't be one "do-it-all" tool. even duolingo doesn't really teach grammar iirc [which really means one can't become fluent tbh]. one kinda has to set up the parts by oneselves, but that's a bit difficult to get used to