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.