You're not going to become a triathlete thanks to training wheels either, but if you don't know how to ride a bike, the training wheels give you a good start. I've learned two languages using Duo Lingo as a supporting tool early in the process. I don't think I would have been able to have conversations so early without it. And for me, having conversations (even poorly) in my target language is the single most important factor for achieving fluency. Yeah, you're not going to become fluent from Duo Lingo alone, but early on in the process it's an essential tool.