For an expert in the language I would agree with your statement. But language is a complex task, like juggling. And you don't start with throwing three balls up

For an expert in the language I would agree with your statement. But language is a complex task, like juggling. And you don't start with throwing three balls up. You start with one, and then when you can handle it you add the second, etc. It's way more boring that way, but it greatly reduces the time needed to get to the more complex topics. I would argue that someone around A2 level in a language (where most hobby learners are) will take 10x longer in learning a song, when applying the method proposed by you. And many people won't ever finish learning one song, because before they finish other topics, songs, activities become more interesting.