Duolinguo tries to follow the input hypothesis. For instance, it barely teaches any grammar but simply asks its users to translate sentences. Unfortunately that's very ineffective. Compared to reading or watching live conversations, the amount of input in unit time on Duolingo is too little. In the meantime the sentences lack sufficient context for Duolingo users to build up intuitive understanding of phrases. Take Duolingo Japanese for English speakers, for instance, it's really hard to learn the meaning and usage of Hiragana words in those short sentences.
That said, I still do about 10 minutes of Duolingo every day, just as a kick start of my daily language-learning routine. It's also an effortless way for me to pick up a few new words on a daily basis. Somehow once I did that, I have more drive to do more comprehensive input by watching Youtube videos or reading some readers.
I completely agree with everything you have mentioned in the first paragraph.
You NEED to consume tens of thousands of words repeatedly used in different contexts for the brain to make those automatic connections. Random sentences do not maintain the context which would have otherwise helped you figure out the possible meaning of some words in the following sentences/paras. That is one of the biggest flaws of any translation method.
The thing is, the input hypothesis all by itself is not enough. It's arguably close to where the modern paradigm of second language acquisition started, but it's not where we still are nearly 50 years later.
For example, one big thing that Duolingo's method completely misses out on is the importance of a rich communicative context. This was implicitly there in Krashen's original monitor model, but wasn't fully appreciated until closer to the turn of the century.
That said, I still do about 10 minutes of Duolingo every day, just as a kick start of my daily language-learning routine. It's also an effortless way for me to pick up a few new words on a daily basis. Somehow once I did that, I have more drive to do more comprehensive input by watching Youtube videos or reading some readers.