I learn the same amount whether I'm in the Ruby league or the much maligned Bronze league and getting promoted to the next league or staying there, gets me uninterested fast. OTOH, if I get a no-ad experience for free, then I'll clock 2000XP every day lol
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Duolingo has an interesting opportunity through some self-awareness. For the consistently high performers, don't give them made-up rewards like promotions to an "Obsidian league", but give them a no-ad experience for free. Use the number of these high performers in marketing.