You will make an excellent passive income for as long as you want in this line of work. In fact, you will even make money on your old blog posts written years ago, depending on the quality and relevance of your content. Your business will always stay open, and you will have cash flowing into your bank account 24 hours a day, 7 days a week, even when you’re asleep or off on a vacation.
You can create a free account on WordPress.com to become familiar with the WordPress dashboard. But when you actually begin designing and building your actual website, you will want to buy domain/hosting and use WordPress.org. Otherwise you will need to do some migrating: https://www.wpbeginner.com/wp-tutorials/how-to-properly-move-your-blog-from-wordpress-com-to-wordpress-org/
The second reason that niche topics will bring you more money is because competition is less fierce. Rather than compete against dozens or even hundreds of other marketers, you’ll only have to compete with a handful. In fact, you may not even compete with the other marketers at all. They may become your friends because you share similar interests. Together you will tackle the market to make even more money.