Yeah that's what upsets me about it. I don't feel like I wasted my time on Duolingo, because I learned two languages while contributing and met a great group of people.
But that incredible knowledge base we created, which was unprecedented in the history of language learning, is gone for future learners. It's completely contrary to Duo's original mission of making learning free and accessible.
The web is getting more and more closed off and we are losing communal knowledge now, instead of gaining it.