But, retention means repetition, so you learn more, right?
Not trying to defend Duolingo but I've been enjoying it for the last 3 years or so. Almost got 1000 day streak and my Spanish is getting better.
My experience is that duolingo is a good component of language learning but is bad as a whole package. I have that, a flash card app, daily word games, and a YouTube channel for a children's TV network in my language. None of them individually would teach me the language, but collectively they reinforce each other and fill in many gaps. Alas, neither innovative language nor lingodeer have the language I want at the moment.
I use DuoCards for flash cards. The word games are FOCLACH (basically wordle in Irish), litreach (guess words from people saying them in 3 Irish dialects) and seafóid (basically Waffle in Irish). The games are all browser based apps so not in app stores, but DuoCards is.
ETA while DuoCards has built in flash cards, I usually make my own based on the words I learn from all the other sources, and get the translation and grammar details from teanglann.ie, tearma.ie, or nualeargais.ie
Duolingo isn’t a good resource for learning a language, it’s focus is user retention
These two statements contradict each other. To learn a language you must practice it every day, week after week, month after month. It's an appropriate application of addictive game mechanics, because our motivation doesn't last long: 1-3 months for most people.
Duolingo might not be the best place to learn some languages (e.g. German), but it can be a very helpful tool for everyday practice. And stuff like streaks, leagues, and other things are rather helpful.