I focus on the "long tail" because as a side-hustle entrepreneur you will never rank on the very popular keywords. You must monetize a lot of lesser-searched keywords, which in aggregate equal one very popular one, and you don't have to defend it obsessively.

You don't necessarily have to get a lot of traffic in order to generate revenue. There are potentially profitable niches that are not monetized at all.

That would be true if the number of topics in the world were constant, but new topics materialize all the time, and the popularity varies on old topics. Articles get old/outdated and are fertile ground for new articles with updated information, etc.

That might be true to rapidly changing fields.

But most businesses work in slowly changing fields.