thats only if you use duolingo exclusively without things like reading news in the language (which is common unfortunately).
its a problem with their specific design using a weird hybrid of spaced repetition with traditional separate lessons. you only learn a couple words for each section but if they decide you know a word they never repeat it again so its easy to forget.
if they made it harder with more open questions and removed combos/perfect lessons to compensate it would be a lot more effective. non linear with multiple paths would also be great, like you decide you want to learn more grammar so you click on that instead of vocab exercises.
the addictive and social pressure parts are the whole point. its giving people motivation to learn and well designed interactive tools are always better than passively reading textbooks [1]. even the ui is made with lots of animation, colors, positive messages to make you feel good every time you get something right.
if streetcomplete added daily progress bars and fireworks every time your edit gets accepted it would probably have a lot more users. ive actually been thinking about making a new anki frontend with this type of addictive ux. that would be more effective and more general than duolingo but lose some of the features like open questions. would need to integrate a llm to fix that.
So, any measure that aims at keeping users engaged (as the duolingo icon for example) should be viewed cautiosly.
Also, specifically with apps with which people provide real data, the more they see it as a game, the more the system will be gamed. When users start to guess, without really confirming it on-site, this leads to outright incorrect data.
What's fine, in my book, is to make the experience more gratifying, more "fun". That's probably what you meant, with firework animation etc.. The progress bars however already fall somewhat in the former category.
By the way, edits are accepted immediately. There is no verification step by the community (just like in wikipedia), all the more important it is that people don't start seeing it as a game first and as a way to contribute to a libre map second.