I'm not criticizing Duolingo, but I'm wondering if language apps are like gym memberships -- people sign up enthusiastically but after the first month, 90% don't or can't continue. Learning a new language as an adult is an incredibly difficult undertaking; much more so than persevering with the gym. The dropout rate must be huge.
I suspect it depends on why you're learning. As you say learning languages is hard, especially as an adult. I couldn't imagine learning a language for "fun", but I know others do. I can easily imagine a lot of them dropping-out and giving up, once they've either learned enough to be useful or decided it is not for them. But me? I've moved from Scotland to Finland. I'm very slowly learning the language. I don't expect…
IMO, you have to have a reason for the language. Mine is that I wanted to read and watch anime/manga/tv/books/games that weren't being translated. Of course, these days they're all being translated faster than I can read them. But it was enough to get started, and I definitely enjoy them more than if I didn't know the original Japanese, too.
But I've tried to learn other languages that I don't have a use for, and I've found it nearly impossible. There just isn't enough motivation.