In general, I agree with your observation, but in terms of Duolingo it does not matter whether language classes would have been better, because I was never going to sign up for them. Letting myself get hooked on another daily word game was a commitment I could actually make, and after plugging away at it with steadily increasing enthusiasm for a little over a year now, I've gained a significant degree of fluency I would not otherwise have. Thus I think that Duolingo offers a genuinely valuable service, not directly comparable to language classes.