In a lot of countries (ex. Belgium, where I'm from) that is not a decent income. Not even close. So you'd have to have a lot of those niche sites to earn a (self-employed) living.
I run two blogs that i kept updating over the years. I don't post frequently, maybe once a month. I earn around 100-150 euro a month from them. It's not much but think of it as pocket money - its a zero effort income - especially if the topics you write about genuinely interest you.
It is supposed to be passive income, so yeah, you have to build a lot of such sites (or a few with high payouts) to replace your regular /salary earnings.
