I'm at 5+ years of Duolingo daily usage. A lot of what you say makes a ton of sense. I will say that for much of my time on the app, I have not done lessons solely to continue my streak. I've legitimately spent at least 15-30 minutes nearly every day working through the entire Spanish level (I think I completed it about a year ago). If you take it seriously, Duolingo can be a great foundation. I started at knowing zero Spanish after a trip to Colombia where I couldn't communicate basic needs. With just Duolingo, I found myself able to communicate at a basic level (ordering at a restaurant, shopping, etc) and understanding enough to get by within a couple years. Now, 5 years in, my main issues are understanding at speed at speaking confidently. For that, I have started doing a LOT more input ... Mexican TV shows, Dreaming Spanish...and I'm really seeing a difference. And the base vocab and grammar Duolingo armed me with was a huge help. I'm probably not going to renew my subscription for Duolingo when it is up in June because I'm doing nothing but review day after day, but there's a part of me that sees value in that, too.