One thing duolingo should he is after every 5 ten minute sessions it should be a ten minute "slow news in the target language" session. There are lots of other options on this line. Really every duolingo style app needs to tell people sooner "you are done, go find something else", either do that by refusing to let them use the app (that will make the VC's unhappy), or give them a different style of study that is more useful than flashcards.