I doubt people who couldn't stick to Duolingo would have stuck to the boring advice HNers have like reading grammar books and watching in-depth Youtube videos t

I doubt people who couldn't stick to Duolingo would have stuck to the boring advice HNers have like reading grammar books and watching in-depth Youtube videos that require much more attention. It seems that people just like ... different things. Some people actually look forward to curling up with a hot, meaty grammar book, for hours on end. And find that both apps and videos (unless done very, very well) just drain their batteries.